What type of quasi-experimental design best fits the following scenario: A school counselor decided to assess effectiveness of a SAT prep course on high school student SAT scores. She compares SAT scores for students who elected to take the SAT prep course to the students who opted out of the SAT prep course.

a. single group posttest-only

b. non-equivalent control group posttest-only

c. single-group pretest / posttest

d. non-equivalent control group pretest-posttest