I think the answer is B. There is no information given about the other router, but by inference it must be set with a higher priority than the one in the question. It would also be set to preempt (by default or by command). Our example is to be the standby. It should take over in the event of a failure of the active rtr, and be preempted when it comes back online.
Answer A, decrementing by 100 seems extreme.
Answer B, set it default, which should be lower than its partner, making it the standby.
Answer C, By decrementing 10, we make it priority 90, but there is no need to preempt, the active one has failed.
Answer D, by making it priority 100 and preempting, would make it active unless its partner had a higher priority.
If the partner has a higher priority already, preempting should be configured on it, not the standby.
Make sense?