C. He's a student. The question asked "what" the boy is. Even though "student" is a person, it is also the name of an occupation; so it is a generic noun which matches the question "what". The others would look like this: "Who is the boy standing there?" ... "He is Tim Green." "What kind of person is the boy standing there?" ... "He's very good." "What is the boy who is standing there doing?" ... "He is reading a book."

C, 这里的wuts 是问的职业，或者身份的意思。 What's the boy standing there? 意为 站在那里的那个男孩是谁（或者是干什么的？） 回答，他是个学生。