A claim is generally not considered credible if

a. it seems likely

b. the claimant is a disinterested party

c. it comes from a source assumed to be credible but who is not known to you

d. the claimant is an interested party

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.