Step 1: A dividing cell cycles through G1 (growth), S (DNA synthesis), G2 (growth and preparation) and M (mitosis), then can either re-enter G1 or leave the cycle altogether.
Step 2: Cells that leave the active cycle move into a separate resting state called G0, where they carry out normal metabolic functions but do not progress toward division, sometimes for the rest of their existence, as seen in mature neurons.
Step 3: Since G0 lies outside the normal G1-S-G2-M progression and represents a non-dividing, resting state, it is specifically termed the quiescent phase.
The correct answer is G0.