N the sentence "Mary caught a frog," the common noun serves as A. the subject of the sentence. B. the verb of the sentence. C. both subject and object. D. the object of the sentence.

D) the object of the sentence. The subject is Mary (the proper noun), as she is who we are talking about. The verb would be caught, since that is the action taking place. The common noun would be "frog," which is the object that the subject is applying the action to.