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.

In the sentence "Mary caught a frog", the common noun serves as: (A) the subject of the sentence.

The common noun in this sentence is 'frog' and we can see that the verb or the action (caught) is being performed on the frog. Therefore frog is the subject of the sentence.