Question

a is a critical point where stop and go signals can regulate the cycle
checkpoint
tumor
growth factor
oncogene

Explanation:

Cell cycle checkpoints are control mechanisms in the eukaryotic cell cycle. They ensure that the cell has met all the requirements before proceeding to the next phase. Stop signals (like DNA damage) or go signals (like growth factor - induced signals) at these checkpoints regulate the cell cycle progression. Tumors are abnormal growths of cells. Growth factors are signaling molecules that promote cell growth, proliferation, etc. Oncogenes are genes that have the potential to cause cancer. The description in the question matches the definition of a checkpoint.

Answer:

A checkpoint is a critical point where stop and go signals can regulate the cycle.