The answer is b. The correlated-groups t-test used fewer participants.
This is because a correlated-groups t-test (also known as a paired-samples t-test) involves comparing two sets of scores that are related in some way, such as before-and-after measurements on the same group of participants. This means that each participant provides two scores, which are then compared to each other.
On the other hand, an independent-groups t-test (also known as a two-sample t-test) involves comparing two sets of scores that are completely independent of each other, such as comparing the test scores of two different groups of students. This means that each participant only provides one score.
So, if both tests have the same degrees of freedom (df = 10), it means that the correlated-groups t-test used a total of 22 participants (11 pairs), while the independent-groups t-test used 20 participants (10 in each group). Therefore, the correlated-groups t-test used fewer participants.