With the Aruba CX switch configuration, what is the Active Gateway feature that is used...

Active gateway is a first hop redundancy protocol that eliminates a single point of failure. The active gateway feature is used to increase the availability of the default gateway servicing hosts on the same subnet. An active gateway improves the reliability and performance of the host network by enabling a virtual router to act as the default gateway for that network. If you have enabled active gateway, VRRP is not required3. Active gateway is similar to VRRP in that routed traffic from the VSX node is sourced from the switch interface MAC and not the virtual MAC address (VMAC). Each active gateway sends a periodic broadcast hello packet to avoid VMAC aging on the access switches. The switch views the active gateway IP as a self IP address3. Active gateway is preferable over VRRP because with VRRP traffic is still pushed over the ISL link, resulting in latency in the network3. Therefore, VRRP and active gateway are mutually exclusive on a VLAN, and answer A is correct.

References: 1: Aruba Campus Access documents and learning resources 3: Active gateway over VSX - Aruba