Personal interviews are good sources of supporting material for speeches because they add:
a. credibility.
b. seriousness.
c. entertainment.
d. purpose.

Option a. Personal interviews are good sources of supporting material for speeches because they add credibility.

Explanation:

When creating and writing a speech it is always important to add good sources of supporting materials. Adding personal interviews is a good way to have a supporting material within your speech that will add credibility to it. A personal interview can be used as a testimony to enhance the main message of the speech, whether because the person being interviewed is considered an expert on the subject of the speech or because it is someone that has firsthand experienced something related to the speech's subject.