Concept:Euglena is a mixotrophic organism; it can switch from photosynthesis to heterotrophic feeding when light is unavailable.Explanation:Euglena contains chloroplasts and photosynthesises in the presence of light.In a dark room, photosynthesis cannot take place.Euglena survives by feeding heterotrophically on organic matter in the pond water.Therefore, it remained alive because it could carry out holozoic nutrition.Answer:A. able to carry out holozoic nutrition