Final answer:
Supermassive black holes feed on gas clouds from the host galaxy, which can be accreted when they get too close to the black hole.
Step-by-step explanation:
The supermassive black hole feeds on a variety of matter from its host galaxy, including gas clouds. These gas clouds, as well as interstellar gas and dust that are part of the galaxy's evolution, can be accreted by the black hole if they venture too close. Quasars themselves are actually active galactic nuclei powered by the energy from material falling into a supermassive black hole, rather than a direct source of food for the black hole. Therefore, the correct answer to what supermassive black holes feed on, besides stars, is c) Gas clouds.