For the most part, protists are very valuable to humans. They form the base of a large portion of aquatic food chains, and without them many ecosystems would collapse. However, some protist species have caused—and continue to cause—profound problems to humans. Of the given answers, the only one not caused by a species of protist is AIDS. AIDS is caused by a retrovirus, which means that it is an RNA virus that uses reverse transcription to replicate itself in a host cell.

The presence of chloroplasts indicate that Euglena is able to photosynthesize. These organelles contain pigments that are necessary for the conversion of light energy to chemical energy. The ability to use non-chemical energy (in this case light) to generate chemical energy is the defining characteristic of autotrophs.

The red eye spot, while it is involved in the complex of organelles that sense light, gives no information about trophic mode: many organisms sense and respond to light, but are not autotrophic.