mushroom is also a plant, but cannot prepare its own food . however green plants can. why it is so?
Let me correct you first that mushroom is not a plant but it is fungus. Mushrooms lack chlorophyll so they cannot prepare their own food(photosynthesis). They are considered as saprophytes that is they obtain their nutrition from dead organic material(dead plants and animals). The plants contain chlorophyll, so they can produce their own food.