Final answer:
A mushroom is a heterotroph because it depends on its environment for nutrients, absorbing organic compounds after breaking them down externally with enzymes.
Step-by-step explanation:
A mushroom is considered a heterotroph because it obtains nutrients from its environment, which is option d. Unlike plants, mushrooms cannot photosynthesize or fix nitrogen from the air. They must rely on organic compounds for carbon and other nutrients. Fungi, such as mushrooms, secrete enzymes to break down organic material in their environment. The resulting simpler compounds are then absorbed by the fungi. This mode of nutrition is similar to animals and differs from autotrophic organisms like plants and algae, which synthesize their own food using sunlight.