In order to display all currently mounted filesystems, which of the following commands could be...

The correct commands to display all currently mounted filesystems are cat /proc/self/mounts and mount. The cat /proc/self/mounts command reads the contents of the /proc/self/mounts file, which is a symbolic link to /proc/mounts. This file contains information about all the filesystems that are currently mounted on the system, as reported by the kernel. The mount command without any arguments shows all the mounted filesystems, as recorded by the mount and umount commands. The other options are incorrect because they do not display the mounted filesystems. The free command shows the amount of free and used memory in the system. The lsmounts command is not a standard Linux command. The cat /proc/filesystems command shows the filesystem types that are supported by the kernel. References:

[LPI Linux Essentials - 2.2 Mounting, Unmounting Filesystems]

How to get the complete and exact list of mounted filesystems in Linux? - Unix & Linux Stack Exchange

4 Commands to List Mounted File Systems in Linux - Linux Shell Tips

How To Show Mounted Devices In Linux Operating System

How to Check if a Filesystem is Mounted in Linux?