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

The correct option is A $G_1$/S
There are three prominent checkpoints in the cell cycle. These are present at late G1, late G2 and late metaphase stage. The checkpoint present at the late G1 phase checks the availability of nutrients, growth factors, DNA damage and allows the commitment of cells to enter the cell division process.
The checkpoint present at the late G2 phase checks if the DNA duplication has been completed and that each chromatid of a chromosome has a copy of DNA. These cells are allowed to enter the M phase.
The checkpoint present at the end of metaphase ensures that those cells with all the aligned chromosomes attached to the spindle fibres, transit to anaphase stage.
Termination or delay in the DNA replication is called stalled DNA replication. The checkpoint $G_1$/S gets activated at this point as it checks for any DNA damage.