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.
