Which configuration command ties the router hot standby priority to the availability of its interfaces?

Which configuration command ties the router hot standby priority to the availability of its interfaces?
A . standby group
B . standby priority
C . backup interface
D . standby track

Answer: D

Explanation:

The standby track interface configuration command ties the router hot standby priority to the availability of its interfaces and is useful for tracking interfaces that are not configured for HSRP. When a tracked interface fails, the hot standby priority on the device on which tracking has been configured decreases by 10. If an interface is not tracked, its state changes do not affect the hot standby priority of the configured device. For each interface configured for hot standby, you can configure a separate list of interfaces to be tracked.

Reference: http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/ release/12.1_19_ea1/confi guration/guide/swhsrp.html