The correct answer is B.
OBJ-3.4: Since the NAS uses a RAID 10 configuration, it is a stripped and mirrored array. When all four 10 TB HDDs are installed, half of the capacity is used by the mirroring functionality of the RAID 10. Therefore, only 20 TB is available as usable storage capacity. This configuration will give the array higher speed/performance (due to the stripping) and increase redundancy (due to the mirroring).