Which sentence uses a verb that agrees with its subject?

a.
behind those walls were a large underground vault.

b.
where is the moose's antlers?

c.
has your parents already written to the manager?

d.
why was kelly laughing?

D. Why was Kelly laughing?

A: Behind those walls is large underground vault.
B: Where are the moose's antler?
C: Have your parents already written to the manager?

The first one is the only one with a verb that agrees with the subject.
The verb was is singular, and so is the noun, Kelly.

Hope this helps :)