Final answer:
The command that shows how much disk space is in use on a Linux system is B. df. It provides a summary of the disk usage for all mounted filesystems, detailing total space, used space, available space, and mount points.
Step-by-step explanation:
The Linux command-line tool that shows you how much disk space is in use is B. df. The command df stands for 'disk free' and when executed, it displays the amount of available and used disk space on your file system. This command provides a summary of the disk usage for all mounted filesystems. The output includes information like the total space, used space, available space, and the mount point of each filesystem.
Other options provided in the question serve different purposes. For instance, top provides a dynamic, real-time view of running system processes. lsof lists open files and the processes that opened them. ps displays the currently running processes. None of these commands give information about the disk space in use as the df command does.