Final answer:
The most common format for a Time Machine backup drive is APFS (Apple File System), which is optimized for flash/SSD storage and has features beneficial for backups. Formerly it was HFS+.
Step-by-step explanation:
The most common format for a Time Machine backup drive used by macOS is APFS (Apple File System), which was introduced with macOS High Sierra. Before that, macOS used HFS+ (Hierarchical File System Plus), also known as Mac OS Extended, which was the default system from 1998 until APFS took over. APFS is optimized for flash/SSD storage and provides strong encryption, space sharing, and file cloning among other features that are beneficial for a backup system. Options B) NTFS and C) FAT32 are file systems commonly associated with Windows operating systems and are not used for Time Machine backups.