When we think about the credibility of a source, we are focusing on whether the source is a. believable b. difficult c. comprehensive d. challenging

When we question the "credibility", or if it's trustworthy, of a source, we are focusing on whether the source is true.
Let's see if any of these terms match "trustworthy" or "true".
A.) Believable. This is a similar term, as it questions legitimacy.
B.)Difficult. This is not an appropriate term to describe 'credibility'.
C.) comprehensive. This has no effect on truth, as truth can be comprehensive and not comprehensive.
D.) Challenging. This is not an appropriate term, as it is similar to difficult.
Your answer is A.) Believable.