Add concreteness, add interest, and to maintain attention from the audience.
a. examples
b. narrations
c. testimonies
d. statistics

The correct answer that would best complete the given statement above would be option C. TESTIMONIES add concreteness, add interest and help maintain attention from the audience. In general, these are termed as supporting materials. Other than testimonies, statistics, examples and narration are also part of the supporting materials. Hope this answer helps.