Which role of protists has the most positive effect on maintaining the plant population in an ecosystem?

1. Protists are a major food source for animal
2. Protists produce approximately 75% of the oxygen on the Earth.
3. Protists decompose dead materials, returning nutrients to the soil.
4. Protists found in the gut of animals assist in digestion.

The answer is Option 3. Protists decompose dead materials, returning nutrients to the soil.

Protists are eukaryotic single-celled organisms which are essential sources of nutrients for many organisms.