Flowers means is that knowledge comes in varied forms. There is academic knowledge, which can be obtained from a higher institution of learning; however, there is also another kind of knowledge, derived from folk wisdom or life experience. Such knowledge can be very useful in helping us navigate life successfully. Mrs. Flowers tells Marguerite that some people who never stepped foot in college are even more intelligent than college professors. This is because they know how to leverage ancient or inherited wisdom to their advantage.

Such people are never at a loss in the face of life's challenges. They are not impeded by their lack of academic knowledge. Instead, their inherited wisdom strengthens them and guides them through life. As a result, they are able to live happy, successful, and secure lives. So, Mrs. Flowers tells Marguerite that ignorance should never be tolerated or rather, the lack of a college education should never be an excuse for one's failure to gain knowledge. Mrs. Flowers insists that folk or inherited wisdom is available to everyone.

She encouraged me to listen to country people's sayings. In those sayings was wisdom collected through the years.

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.