The answer is b) fungi.
Fungi are primarily heterotrophic organisms, meaning they obtain their food by absorbing nutrients from their environment. They do this by breaking down complex organic materials, which can involve decomposing dead matter or forming symbiotic relationships with other organisms.
On the other hand, Protista and Plantae include groups that can be autotrophic, such as algae (in Protista) and plants, which produce their own food through photosynthesis. Therefore, while fungi are indeed heterotrophic, not all members of the other two kingdoms fall into this category.