When cell has stalled DNA replication fork, which checkpoint should be predominantly activated?

The correct option is A

Explanation of the correct answer:

1. The three main checkpoints are – G1 checkpoint (restriction or start or major checkpoint); the G2/M checkpoint; and lastly metaphase-to-anaphase transition (spindle checkpoint).
2. ​The following checks are done at the G1 checkpoint, at the G1/S transition:
3. Size. Is the cell large enough to divide?
4. Nutrients. Does the cell have enough energy reserves or available nutrients to divide?
5. Molecular signals. Is the cell receiving positive cues (such as growth factors) from neighbours?
6. DNA integrity. Is any of the DNA damaged?
7. If all the situations are suitable for the cell, the DNA undergoes replication and a replication fork is formed.
8. If there is any fault found during the check, cell stops the DNA replication and there is stalled DNA replication fork.

Explanation of the incorrect answers:

Option B:

1. at the G2/M transition, the cell will check:
2. DNA integrity. Is any of the DNA damaged?
   
3. DNA replication. Was the DNA completely copied during the S phase?​
4. If any mistake in the copied DNA is found the cell does not enter M phase.
5. Otherwise, the cell enters the M phase.
6. Since, the cell has already undergone replication there is no stoppage of DNA replication.

Option C:

1. During M phase, the checkpoint ​is the transition stage from metaphase to anaphase.
2. The arrangement of the spindle fibres is checked.

Option D:

1. The checking of the discrepancy in the DNA before replication is done in the G1/S checkpoint.
2. It is checkpoints G2/M and M come after the DNA has been replicated completely.

Final answer: At $G_1$/S cell has stalled DNA replication fork, which checkpoint should be predominantly activated.