Certain species of which one of the following organisms are well known as cultivators of fungi?

The correct option is Ant.

Certain species of ants, most notably the leafcutter ants (belonging to the tribe Attini), are famous for their highly evolved, mutualistic relationship with fungi. Instead of feeding directly on the leaves they harvest, these ants use the vegetation as a substrate to cultivate specialized fungal gardens (specifically of the family Lepiotaceae) within their nests.
The ants actively clear competing microbes, apply antibiotics to protect the crop, and feed the fungus. In return, the fungus produces specialized, nutrient-rich structures called gongylidia, which serve as the primary food source for the ant colony.