reliable, honest, truthful
Credible means something that is believable or capable of being believed; believable. For example, credible evidence refers to evidence that is worthy of belief or trustworthy. In the question this means that the sponsor denial or the explanation of it, is believable or trustworthy.
The most likely definition of the word "credible" is believable or trustworthy. It refers to something that is considered reliable or deserving of confidence.
The prefix on the word "incredible" is "in-" which means "not."
Believable, unquestionable, undoubted, palpable...
Shakespeare did not use the word "trustworthy" but he did use the word "trusty" a lot which means the same.
Reliable.
The word "trustworthy" means "worthy of trust". Depending on the context, it could have the synonyms reliable, dependable, steadfast, or loyal.
Trustworthy is an adjective.
The Sanskrit word for trustworthy is "विश्वासनीय" (Viśvāsanīya).
The word "plausible" means something that is believable or credible. The suffix "-ible" in plausible comes from Latin and means "able to be."
The German word for trustworthy is vertrauenswürdig.
Saying that your book report blew out the car window is a flimsy excuse.