Answer: d)
Explanation: Those are the opinions that are missing in you question -
a. subgrouping
b. prejudice
c. stereotyping
d. discrimination
Dr. Charles is discriminating against African American students because he doesn't give them time, attention and help as he is giving to other students.
It is also discrimination that is prejudiced, he is classifying his students by the group of people that they belong to.
Examples of discrimination are judgment and classification by color, gender, sexual orientation, pregnancy, religion, material status or age and in this case Dr. Charles is showing discrimination by race and national extraction.