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Comprehensive and Detailed Explanation (AWS Verified – 250–300 Words)

When an Amazon S3 bucket is configured with default encryption using SSE-KMS, AWS Key Management Service (KMS) plays an active role in every PutObject request. Although the IAM user already has the s3:PutObject permission, this alone is not sufficient when SSE-KMS is enabled.

According to AWS documentation, when a client uploads an object to an S3 bucket encrypted with SSE-KMS, Amazon S3 must call AWS KMS on behalf of the caller to generate a unique data encryption key. This operation requires the kms:GenerateDataKey permission on the KMS key used for encryption. If this permission is missing, the PutObject request fails with an Access Denied error—even though S3 permissions appear correct.

AWS explicitly states that IAM principals uploading objects to SSE-KMS–encrypted buckets must be allowed to use the KMS key. At a minimum, the following KMS permissions are required:

kms:GenerateDataKey

(Implicitly) access allowed by the KMS key policy

Option C correctly resolves the issue by granting the IAM user permission to generate a data key via AWS KMS, enabling successful encryption during the upload process.

Option A is invalid because s3:EncryptionConfiguration is not required for object uploads.

Option B is unnecessary because the IAM user already has s3:PutObject.

Option D is incorrect because ACLs are not used to control KMS encryption permissions and are discouraged by AWS best practices.

Sensitive environment variables in Lambda should be protected using encryption helpers integrated with AWS KMS. Lambda environment variables are encrypted at rest, and when using a customer managed KMS key, teams can control the key policy, access, and rotation for their own microservices—matching the policy requirement that each team has “full control” over key rotation. AWS managed keys do not provide the same level of customer control over lifecycle and rotation decisions.

Therefore, the correct approach is to create customer managed KMS keys (one per team/service boundary or per microservice, depending on governance) and configure each Lambda function to use the relevant CMK for encrypting its environment variables. The Lambda execution role must have permission to use the key for decryption at runtime, typically requiring kms:Decrypt (and sometimes kms:DescribeKey depending on tooling). This is captured by Option B.

Option A and D use AWS managed keys, which do not satisfy the requirement for each team to control rotation. Rotation control (and policy ownership) is a key differentiator of customer managed keys.

Option C is not correct because the Lambda execution role does not need kms:CreateGrant and kms:Encrypt for decrypting environment variables at runtime. Over-permissioning increases risk. The Lambda service handles encryption of environment variables; the function runtime principally needs the ability to decrypt.

Amazon API Gateway is a service that enables developers to create, publish, maintain, monitor, and secure APIs at any scale. Amazon CloudWatch is a service that monitors AWS resources and applications. API Gateway provides several CloudWatch metrics to help developers troubleshoot issues with their APIs. Two of the metrics that can help the developer troubleshoot the issue of API Gateway timing out are:

IntegrationLatency: This metric measures the time between when API Gateway relays a request to the backend and when it receives a response from the backend. A high value for this metric indicates that the backend is taking too long to respond and may cause API Gateway to time out.

Latency: This metric measures the time between when API Gateway receives a request from a client and when it returns a response to the client. A high value for this metric indicates that either the integration latency is high or API Gateway is taking too long to process the request or response.

CloudFront is a content delivery network that caches objects at edge locations to reduce latency and origin load. When the developer updates static artifacts in the origin S3 bucket, CloudFront may continue serving cached versions until the objects expire based on cache-control headers or the distribution’s TTL settings. That is why the developer sees the updated files in S3 but not on the site.

The standard fix is to perform a CloudFront cache invalidation for the updated object paths (for example, /app.js, /styles.css, or /* if needed). An invalidation forces CloudFront edge locations to remove the cached objects so the next viewer request fetches the latest version from the S3 origin.

Option C directly addresses this behavior and is the correct operational practice when cache TTLs would otherwise delay updates.

Option A (Object Lock) is for WORM retention/compliance and has nothing to do with CloudFront cache refresh.

Option B might change what exists in S3 but does not guarantee CloudFront will stop serving cached content; the cache can still serve objects even if deleted from the origin (until it revalidates/refreshes).

Option D is unnecessary; the origin does not need to change to fetch updated content.

Therefore, invalidate the CloudFront cache after deployment to ensure the new artifacts are served.