Even justified actions have explanations. The stimulus says EVERY human action potentially has an explanation, given sufficient knowledge of the causes. Justification doesn't make an action's causes vanish; the donor in the example still has causes for the donation, whether or not those causes match up with the reasons.

This reverses the conditional. The stimulus says rational → reasons are an essential part of the causes. (B) says reasons among the causes → rational. Even if reasons happen to appear in the causal mix, they might not be ESSENTIAL (the stimulus is specific about that word). The donor in the example might have been partly motivated by the reasons and mostly motivated by impressing someone; reasons would be among the causes without being essential, and the action wouldn't count as rational.

Too strong. The stimulus says many psychologists believe justification "often" forms no part of the explanation. "Often not" isn't "never." The psychologists' view leaves room for justification sometimes being part of the explanation. (C) takes "often" and ratchets it up to "never," which the stimulus doesn't support.

The stimulus says that with sufficient knowledge it would be possible to give an accurate description of an action's causes. So in principle every action's causes can be discovered. We have no reason to think some actions have causes that can't be discovered.

This follows from the principle. If any action is rational, the principle requires the reasons for that action to be an essential part of its causes. So for that action, reasons are among the causes. If reasons are never causes anywhere, no action could be rational; so as soon as one rational action exists, reasons must sometimes be causes. That's exactly what (E) claims.