Refer to the exhibit. An engineer must modify the existing configuration so that R2 can take over as the primary router when serial interface 0/0.1 on R1 goes down. Which command must the engineer app

The `standby 100 preempt` command configured on R2 enables HSRP preemption for group 100. This feature is critical because it allows R2 to take over as the active router if its priority becomes higher than R1's, which would happen when R1's tracked interface goes down and its priority is decremented.

`standby 100 priority 100` sets R2's priority, which is part of HSRP configuration, but it does not, by itself, allow R2 to take over as active if R1's priority drops; preemption is required for that.

`standby 100 track 26 decrement 10` would configure R2 to track an interface and decrement its own priority, which is not the mechanism for R2 to take over when R1's interface fails.

`track 26 interface Serial0/0.1 line-protocol` is a global tracking command and would typically be configured on R1 to monitor its own interface, defining what is tracked but not the HSRP behavior on R2.