pgrep looks through the currently running processes and lists the process IDs that match the selection criteria to stdout. All the criteria have to match. pkill is a tool that looks up or signals processes based on name and other attributes. Pkill will send the specified signal (by default SIGTERM) to each process instead of listing them on stdout.

Answer option A is incorrect. The top command is used to produce a frequently-updated list of processes. By default, the processes are ordered by percentage of CPU usage, with only the "top" CPU consumers shown. This command shows how much processing power and memory are being used, as well as other information about the running processes. The following figure demonstrates the output of the top command:

Answer option C is incorrect. The sort command is used to print the lines of its input or concatenation of all files listed in its argument list in sorted order. Sorting is done based on one or more sort keys extracted from each line of input. By default, the entire input is taken as the sort key. The blank space is taken used as the default field separator in the output of the sort command.

The -r flag, however, reverses the sort order. For example, the ls -s | sort -nr command will sort the directories of the root directory in reverse order of size.