Default BGP keepalive timer in Junos is 30 seconds

The default keepalive time for BGP is 30 seconds, which means that BGP peers send keepalive messages to each other every 30 seconds to verify the liveness of the BGP session. If no keepalive message is received from a peer within the hold time, which is 90 seconds by default, the peer is considered to be unavailable and the BGP session is closed. The keepalive time and the hold time can be configured manually for each BGP peer or peer group, but they must match on both ends of the BGP session. The keepalive time should be one third of the hold time for optimal performance. Therefore, option C is correct and the other options are incorrect. References:  BGP keepalive messages and hold time - Juniper Networks

According to the Juniper documentation1, a native VLAN is a VLAN that carries untagged traffic on a trunk port. A Wi-Fi access point may need a native VLAN to associate untagged traffic with a specific VLAN, such as the management VLAN. On Juniper EX Series switches, you can enable a native VLAN on a switch port by setting the interface-mode to trunk and specifying a native-vlan-id. For example, the following configuration enables a native VLAN with ID 1 on the switch port ge-0/0/12:

set interfaces ge-0/0/1 unit 0 family ethernet-switching interface-mode trunk

set interfaces ge-0/0/1 unit 0 family ethernet-switching vlan members all

set interfaces ge-0/0/1 unit 0 family ethernet-switching native-vlan-id 1

The other options are not correct because they do not address the requirement of untagged traffic. VLAN tagging is the process of adding a VLAN identifier to a frame, which is not needed for untagged traffic. Flexible VLAN tagging is a feature that allows multiple VLAN tags on a single interface, which is not relevant for this scenario. Voice VLAN is a feature that allows voice traffic to be separated from data traffic on a switch port, which is not related to untagged traffic or Wi-Fi access points. References: Understanding Native VLANs on Trunks | Junos OS, Configuring a Native VLAN on a Trunk Port | Junos OS