The correct answer is A.
According to the information provided, Euglena are protists which are both heterotrophs and autotrophs. They can ingest food through phagocytosis, which means that they are heterotrophs. However, they contain chloroplasts and a structure acting as a light receptor. This means that Euglena can use light energy to produce chemical energy, through photosynthesis.
If sunlight is limited, then the photosynthetic process cannot occur and the Euglena will ingest food.