Answer:
C. Adverb phrase.
Step-by-step explanation:
1. The librarian read a story to the children.
2. The librarian clearly and cheerfully read a story to the children.
The first sentence is just a simple sentence. Sentence 2 gives a further description of the sentence. The addition of the phrase "clearly and cheerfully" is an adverb phrase for it gives the 'how' of the verb 'read'. Adverb phrases give a better emphasis on how, when, why the verb is done. So, in the sentence, the adverb phrase "clearly and cheerfully" gives more emphasis on the action "read". Thus, the answer is the adverb phrase "clearly and cheerfully" is added in the first sentence to make the second sentence.