Answer: a. within-subjects design
Explanation:
A within subject design is an approach in which same group of subjects are used for every trial of the experiment and the each subject is exposed to every treatment and condition. Each subject is exposed to the various independent variables these variables are controlled by the experimenter. According to the given situation, the experiment is conducted on the same group of students and each received the psychometric test at different intervals of the study so the given situation is an example of within-subject design.