Final answer:
A mushroom is the organism listed that is a heterotroph because it relies on organic compounds produced by photoautotrophs for its energy.
Step-by-step explanation:
The organism among the given options that is a heterotroph is the mushroom. Heterotrophs are organisms that cannot synthesize their own food and must rely on other sources of organic compounds for their energy needs. As detailed in the reference material, plants, algae, and cyanobacteria are capable of performing photosynthesis and are therefore considered photoautotrophs or self-feeders using light. On the other hand, organisms like animals, fungi (which include mushrooms), and most other bacteria are heterotrophs because they rely on the organic compounds produced by photoautotrophs. Thus, this eliminates cyanobacteria, oak trees (a plant), and algae, all processes of photosynthesis, leaving the mushrooms as the correct answer.