Question

Why should historians question Edward Ennis?

Answer 1

I believe the answer is: b.  Ennis alerted his superiors to the existence of secret evidence but, when they ignored it, Ennis did not make the evidence public. He remained silent.
Withholding information usually is not the actions that taken by the honest individuals. Ennis actions indicate that He could receive something from the superiors that make him feel justified to stay silent rather than showing all the evidence to the public.