Each of the participants in the study was asked to complete surveys that assessed their level of stress, but only 20 percent returned them by the deadline.
Correct: D
The subject 'each' is singular, so the pronoun 'them' must not refer to singular 'it.' However, the surveys are plural ('surveys'), so 'them' is correct. The confusion arises from '20 percent of them' incorrectly using 'of them' after '20 percent.' The correct pronoun 'it/its' would only apply if 'each' was pluralized.