A noun is a person, place, or thing. The nouns in this sentence are the people, the places, and the things, meaning the answer is D. Boy and girl are people, school is a place, and hands are things.
D) According to Wanda Yamaguchi, "Making friends takes work."
Your answer is B., because it is unidentifiable, so if they don't know the source, then why cite it right? :P :D So anyways, that's why your answer is B. =)
I hope I helped! =D