Final answer:
The correct word is 'mononuclear,' which applies the prefix mono- to describe a cell having a single nucleus, as seen in monocytes.
Step-by-step explanation:
The word that uses the prefix mono- to mean pertaining to one nucleus is mononuclear. It indicates a cell with a single nucleus, which is a membrane-bound structure that houses the cell's DNA. Examples of mononuclear cells include monocytes, which are a type of leukocyte found in the blood responsible for the phagocytosis of pathogens and damaged cells.