Which of the Following Scenarios Exemplifies a Case Study as a Method

Which of the following scenarios exemplifies a case study as a method of descriptive research in psychology?

A) Professor McGonagall asks all her psychology students to complete a mood survey for extra credit.
B) Dr.Fiennes conducts a series of interviews over the course of a year with a student diagnosed with an anxiety disorder.
C) Mr.Pierre, a researcher, observes the study habits of students in the library versus students in the cafeteria.
D) Mrs.Dmitri, a teacher, assigns half her class to computerized instruction and the rest to the traditional classroom.