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

Since the common noun here is 'frog', the correct answer is D, the object of a sentence.
A cannot be correct because the subject is 'Mary' which is a proper noun.
One word cannot be simultaneously a subject and an object, so B isn't correct.
In C, the verb is actually 'caught', verb cannot be a noun. 

B. metaphor is the answer