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.
