Testimonials from satisfied customers would be one way that Austin could establish:

A) persuasive ability.
B) source acceptance.
C) credibility.
D) empathy.
E) active listening.

Credibility

The quality of being trusted and believed in; the ability to inspire confidence.

Source Acceptance

The extent to which the information provided by a source is accepted as credible or convincing by a recipient.

Empathy

The capacity to understand or feel what another person is experiencing from within their frame of reference, essentially placing oneself in another's shoes.