The correct answer is B. Paired t test . The key phrase in the question is that the team measures the same hard drives before and after repair. In CSSBB statistics, a paired t test is used when data are taken in pairs , with a difference calculated for each pair. The CSSBB source states that the paired t test “tests the difference between 2 sample means” when the “data is taken in pairs,” and that a different d is calculated for each pair.

This is exactly a before-versus-after situation on the same units, so the observations are dependent , not independent. The supplemental material also explains paired sampling as pre-test/post-test measurements on the same element , which confirms that this is not an ordinary two-sample independent t test. ANOVA is used for comparing three or more means, and the F test is used to compare variances. Therefore, the proper CSSBB-aligned test for performance differences on each drive is the paired t test .