Cells will usually divide if they receive the proper signal at a checkpoint in which phase of the cell cycle? see concept 12.3 (page 244)

The correct answer is the interphase.

The cell cycle consists of three phases: the interphase (G1, S, G2) the mitotic (M) phase, and cytokinesis. The first checkpoint of the cell cycle is G1 checkpoint which is between G1 and S phase when cell checks its size, nutrients, molecular signals, DNA integrity. The second checkpoint is G2, just before the mitotic phase, when cell checks DNA integrity and DNA replication. Those two checkpoints are the ones before division, other like M checkpoint is during the mitosis.