A virtual tape library (VTL) is a disk-based backup system that emulates a tape library. It provides faster backup and recovery than traditional tape systems, and it can be integrated with data deduplication and replication technologies to enhance disaster recovery. A VTL can also be replicated to an offsite location for additional protection. A VTL is the best disaster recovery solution for an environment where data virtualization is used, because it can handle large volumes of data, support multiple backup applications, and provide consistent performance.

Onsite disk-based backup systems (A) are not the best disaster recovery solution, because they are vulnerable to the same risks as the primary data center, such as fire, flood, power outage, or sabotage. Tape-based backup systems (B) are not the best disaster recovery solution, because they are slow, prone to errors, and require manual intervention. Redundant array of independent disks (RAID) (D) is not a backup system, but a storage technology that improves performance and fault tolerance by distributing data across multiple disks. RAID does not protect against data corruption, human error, or malicious attacks.