D is incorrect. It does not establish the new mount point once the mirrored pool is split.
Not C as -n is for a dry run
and not B as it is dettaching vdev

The correct answer is A (however, it should be done using the -R option or there will be a conflict on mount points)

# zpool split -R pool1 pool2 c4t0d0 c4t2d0
# zpool import pool2
# zfs set mountpoint=/dev_data pool2/prod_data