Choice "D" is correct. An off-site mirrored web server allows the off-site web server to take over almost immediately in the event of a disaster, thereby providing nearly uninterrupted service and allowing for business continuity.
Choice "c" is incorrect. Backing up the server database daily will provide a second copy of the data; however, by storing it in the same location, it would also be at risk in the event of a natural disaster and would not be an effective business continuity strategy.
Choice "a" is incorrect. Records are stored off-site for many reasons and while it is important to have these records in the event of a disaster, it would take days or weeks to upload it into a new system, making it a poor strategy for business continuity.
Choice "b" is incorrect. RAID (Redundant Array of Independent Disks) is often used for disk storage. The basic idea of RAID is to combine multiple inexpensive disk drives into an array of disk drives to obtain performance, capacity and reliability that exceed that of a single large disk drive. This is implemented at the original location for disk storage and is not considered a disaster recovery and business continuity option.