Which of the following is needed to be enable back the role of active in HSRP?
In the Hot Standby Router Protocol (HSRP), two or more routers work together to present the appearance of a single virtual router to the hosts on a LAN. One of the routers is elected as the active router, and the others are in standby mode, ready to take over if the active router fails.
In order to enable the role of active back, the HSRP standby router with the highest priority will preempt the active router when it becomes available again. Therefore, the answer is A. preempt.
Here's a brief explanation of the other options:
B. priority: This is used to determine which router becomes the active router when all routers are started or restarted at the same time, or when the current active router fails. The router with the highest priority becomes the active router.
C. failover: This is the process of switching from the active router to a standby router when the active router fails.
D. revert: This is not an HSRP command, but it may be used in other contexts to describe a process of returning to a previous state or condition.