In the sentence "mary caught a frog," the common noun serves as
a. the subject of the sentence.
b. the predicate of the sentence.
c. the object of the sentence.
d. both subject and object.

C. The object of the sentence.

First, let's identify the common noun: 
Mary - wrong, cause it's a name.
Frog - yes, it's a common noun.

Second, let's see its role in the sentence: 
It comes after a verb, showing that it must be either an object of a predicate.
Predicate - wrong, cause "caught" is not a linking verb.
Object - correct answer.

---> Final C.