During what phase of the cell cycle do some cells exit the cycle in an indefinite period of rest?

The G0 phase is the correct answer as it represents a state in which cells exit the active cell cycle and enter a quiescent or resting state. During G0, cells do not replicate their DNA or divide, and they may remain in this phase for an indefinite period. This can occur in response to various environmental signals, such as nutrient availability or growth factors.

In contrast, the G1, G2, and S phases are all active phases of the cell cycle where specific preparations for cell division take place. In the G1 phase, cells grow and conduct their normal functions while preparing for DNA replication. During the S phase, DNA synthesis occurs, leading to the duplication of chromosomes. The G2 phase follows this, where the cell prepares for mitosis by further growth and checks for DNA replication errors. Thus, it is in the G0 phase where cells can remain metabolically active yet indefinitely non-dividing, distinguishing it from the other active phases of the cell cycle.