Final answer:
The average length of the M phase in rapidly dividing human cells with a 24-hour cell cycle is about 1 hour, as the M phase lasts approximately 0.5 hours. Option A is correct.
Step-by-step explanation:
The average length of the M phase in a rapidly dividing human cell with a 24-hour cell cycle is about 1 hour. In detail, the M phase, which includes both mitosis and cytokinesis, lasts approximately one-half hour (0.5 hours) when mammalian cells are grown in culture.
For context, in a typical 24-hour cell cycle for fast-dividing human cells, the G₁ phase lasts around 9 hours, the S phase is about 10 hours, the G₂ phase is approximately 4.5 hours, and the M phase is around 0.5 hours.
Therefore, the correct answer to the question of the average length of the M phase is A: about 1 hour.