Animals would be out, because they do not produce their own food.
Fungi decompose other matter and break it down to get glucose and other minerals. They do not produce food either.
Protists are the 'odds and ends' kingdom. Some protists are plant-like, animal-like, or fungus-like. So it wouldn't contain ONLY producers.
The answer to this would be Plants because they generate their own food through the process of photosynthesis, creating glucose in photosynthesis and breaking it down for energy in cellular respiration.