Question

A claim is generally not considered credible if

Answer 1

Credible means easy to believe. If something is likely; why should it not be credible?

also if it comes from a credible source, I would assume that it's credible.

Now, we're left with the choice between interested and disinterested party: among the two the interested party is more likely to make false claims, as they can benefit from them: so I think that answer d. the claimant is an interested party  is the correct one.