P notes that while most species can communicate with each other, but not all communication is language- and that human communication is language, which is a core trait that makes us human.
S agrees that not all communication is language, but questions why some kinds of evolved communication (like apes) are not language.
S assumes that the professor thinks that highly evolved forms of communication outside of humans are not language. However, the Professor never said that. She only said that not all communication is language, and that human communication is certainly language. She never said which forms of nonhuman communication are or are not language, only that not all are language.
In other words, the student has mistakenly assumed that the professor thinks that only human communication is language, which is a good match for D.