The filesystem that is compatible with the greatest number of operating systems is FAT32. FAT32 stands for File Allocation Table 32-bit, which is a filesystem that organizes data into clusters or groups of sectors on a storage device, such as a hard disk or a flash drive. FAT32 uses a 32-bit table to keep track of the location and status of each cluster. FAT32 can support volumes up to 2 TB and files up to 4 GB in size. FAT32 is compatible with most operating systems, such as Windows, Linux, Mac OS, Android, etc., as well as most devices, such as cameras, printers, game consoles, etc. FAT32 is one of the oldest and simplest filesystems, but it also has some limitations and drawbacks, such as fragmentation, waste of space, lack of security features, etc. ext4 is not the filesystem that is compatible with the greatest number of operating systems, but rather a filesystem that is mainly used by Linux operating systems. ext4 stands for Fourth Extended Filesystem, which is a filesystem that organizes data into blocks or groups of sectors on a storage device. ext4 uses an inode table to keep track of the location and attributes of each file or directory. ext4 can support volumes up to 1 EB and files up to 16 TB in size. ext4 has many features and advantages over FAT32, such as journaling, extents, subdirectories, encryption, etc., but it also has limited compatibility with other operating systems, such as Windows or Mac OS. NTFS is not filesystem that is compatible with greatest number of operating systems, but rather filesystem that is mainly used by Windows operating systems. NTFS stands for New Technology File System, which is filesystem that organizes data into clusters or groups of sectors on storage device. NTFS uses Master File Table (MFT) to keep track of location and attributes of each file or directory. NTFS can support volumes up to 256 TB and files up to 256 TB in size. NTFS has many features and advantages over FAT32, such as journaling, compression, encryption, security, etc., but it also has limited compatibility with other operating systems, such as Linux or Mac OS. HFS is not filesystem that is compatible with greatest number of operating systems, but rather filesystem that is mainly used by Mac OS operating systems. HFS stands for Hierarchical File System, which is filesystem that organizes data into blocks or groups of sectors on storage device. HFS uses catalog file to keep track of location and attributes of each file or directory. HFS can support volumes up to 2 TB and files up to 2 GB in size. HFS has some features and advantages over FAT32, such as resource forks, aliases, etc., but it also has some limitations and drawbacks, such as fragmentation, waste of space, lack of security features, etc. HFS also has limited compatibility with other operating systems, such as Windows or Linux. References: The Official CompTIA IT Fundamentals (ITF+) Student Guide (Exam FC0-U61), Chapter 4: Operating System Fundamentals1