Which command is used to count the number of lines in a file?

The command used to count the number of lines in a file is "wc," which stands for word count. When used with the "-l" option (i.e., wc -l), this command specifically counts and displays the number of lines in the provided file. It is a versatile command that can also count words and characters when used with different options, but for line counting, "-l" is the appropriate flag.

Other commands mentioned do not serve the same primary purpose. For instance, "count" is not a standard Linux command and would yield an error if attempted. The "grep -c" command counts the number of lines that match a specific pattern or expression rather than counting all lines; it is more suited for searching. The "cat -n" command, on the other hand, displays the contents of a file with line numbers but does not provide a count of total lines directly. Thus, the correct choice for counting lines in a file is indeed "wc."