The best answer to this question is C -- Mrs. Flowers is telling Marguerite that a person who does not go to school (more importantly, someone who is unable to go to school) is not necessarily stupid. They can be just as smart as a college professor.
D is given as the answer that is supposed to trip you up. Mrs. Flowers does talk about college professors, but she does not say they are ignorant. She simply says that some uneducated people might be as smart as the college professors.
This is one of the "lessons in living" that Mrs. Flowers gives Marguerite, beginning soon after Marguerite is sent to Stamps to live.