Which agreement rule applies to a singular collective noun functioning as a unit?
a. use a singular verb
b. use a plural verb
c. make the verb agree with the subject closest to the verb
d. use a plural verb if the collective noun uses and

The answer is letter A.  
 Collective nouns could be taken as singular or plural. This depends on how each is used in a sentence. A singular collective noun is a word that functions as a single unit.
 
For example: The play’s cast is currently rehearsing. The word “cast” is a singular subject. Although there are many members of the cast, they still function as one unit.

The plural collective noun on the other hand indicates many units and thus takes a plural verb.