In general terms, normally a cell in your body crosses a checkpoint in the cell cycle when
a. it receives a "go-ahead" signal directly from your brain
b. concentrations of proteins in the cytoplasm reach the appropriate level
c. it receives a "go-ahead" signal from your external environment
d. concentrations of dna in the nucleus reach the appropriate level
e. a mutation in the dna occurs

B. Concentrations of proteins in the cytoplasm reach the appropriate level

Explanation:

We can say that the cell cycle checkpoints is an important cell cycle control. In this regard, it is important to note that within a cell, the passage from one phase to another is controlled by regulatory factors that are characteristically proteinic and act at the so-called cell cycle checkpoints.

A cell passes the first checkpoint when protein concentrations in the cytoplasm reach the appropriate level.

a chair, a bed, and a toothbrush

explanation: