Answer: a) Autotrophs supply food for the heterotrophs.
Explanation: Autotrophs are organisms that can produce their own food, while heterotrophs need to ingest its energy source.
Plants are autotrophs because, through photosynthesis, they can produce glucose and oxygen, a source of energy.
Deer is a heterotroph which feeds itself by eating plants. Bears are also heterotrophs who get their energy source through ingesting deers. So, in the food web, plants, who produce their own food, is the energy source of deer, which, in its turn, is the food source of a bear.
In conclusion, autotrophs are the primary food supply in the food web, supplying food for the heterotrophs.