Which of the following commands finds all lines in the file operating-systems.txt which contain the term linux, regardless of the case?

igrep is not a standard Linux command and does not exist in typical distributions.

less is a file pager for viewing file contents interactively; the -i flag in less affects case sensitivity only within its interactive search prompt, not as a command-line file search tool.

grep is the standard Unix utility for searching text patterns in files. The -i flag tells grep to ignore case distinctions, so it will match 'linux', 'Linux', 'LINUX', and any mixed-case variant within operating-systems.txt.

cut is used to extract specific fields or character ranges from lines of text, not to search for string patterns.

grep --sort is not a valid grep option; grep has no --sort flag and this command would produce an error.