The correct answer is C) Lichen. Lichen consist of two microbial colonies, one photosynthetic, the other not. The photosynthesizing microbes make food for the other microbes.
Mushrooms do not photosynthesize, their only function is to spread fungus spores.
Yeast do not photosynthesize, they either respirate or ferment depending on whether the environment is aerobic, or anaerobic.
Slime mold does not photosynthesize, nor does it exist symbiotically with other organisms.
Although some protozoans do photosynthesize, not all of them do. And the relationship between different protozoa tends to be producer/consumer, not symbiosis.