Answer:
Credible, is the right answer.
Explanation:
A credible speaker is one whom the public understands to be responsible, knowledgable, energetic and trustworthy. He is the one who has extensive knowledge about his area of interest or subject. While giving a speech he may refer to some sources or give some references which makes the audience to believe in his/her speech. Therefore, the believability of a speaker is highly dependent on credible supporting materials.
