What do cells check at the G₂/M checkpoint?

The G₂/M checkpoint plays a crucial role in ensuring that the cell is fully prepared to undergo mitosis. At this checkpoint, the primary focus is to verify that DNA replication has been completed successfully and that the DNA is free of damage. This is critical because any errors in DNA can lead to mutations, genomic instability, or cancer development if the cell proceeds to divide without rectifying these issues.

By confirming that DNA replication is complete, the cell can ensure that both daughter cells will have the correct genetic material post-division. Additionally, the detection of DNA damage at this stage allows for mechanisms to repair any issues before moving forward into mitosis, thus safeguarding against the propagation of defective DNA.

While other checkpoints and stages in the cell cycle do monitor different aspects such as growth signals, spindle fiber attachment, and chromosome separation, they do not directly address the integrity and completeness of DNA replication like the G₂/M checkpoint does. This makes option B the most appropriate and relevant choice in the context of this particular checkpoint's function.