Question

In the sentence "Mary caught a frog," the common noun serves as

Answer 1

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 answer: C.