Final answer:
Interphase is the period when the cell is not dividing and is preparing for cell division by growing and duplicating its DNA. It is the longest phase of the cell cycle and comes before the stages of mitosis. Therefore, the correct option is C.
Step-by-step explanation:
The period of time when the cell is not dividing is called interphase. During interphase, the cell grows, duplicates its DNA, and prepares for cell division. It includes the G₁, S, and G₂ phases. This phase constitutes the majority of the cell’s life cycle and functions to prepare the cell for mitosis, which is the next stage in cell division. Interphase is characterized by the cell's preparation for division, where it increases in size and duplicates its DNA but does not yet begin the active process of cell division that is seen in the stages of mitosis (prophase, metaphase, anaphase, and telophase).