Answer: The main source of radio exposure for indie rock in the 1990s was college radio.
Explanation: College radio stations were especially important for indie rock bands, as they provided an alternative to the mainstream music played on commercial radio stations. College radio stations provided indie rock bands with a platform to reach a wider audience, and many of these bands gained a large following as a result. College radio stations also provided an outlet for independent labels to promote their music, which allowed indie rock to gain more exposure.