Explanation
The cut command is used to extract selected fields from each line of a file. The -d option specifies the delimiter that separates the fields, and the -f option specifies the fields to print. The /etc/passwd file contains information about the users on the system, and each field is separated by a colon (:). Therefore, cut -d : -f 1,4
/etc/passwd will print the first and fourth fields of each line, which are the username and the primary group ID respectively. The other commands are either invalid or do not perform the desired task. The fmt command is used to reformat paragraphs of text, but it does not have a -f option. The split command is used to split a file into smaller files, but it does not have a -c option. The paste command is used to merge lines of files, but it does not have a -f option. References:
* LPIC-1 Exam 101 Objectives, Topic 103: GNU and Unix Commands, 103.3 Perform basic file management
* LPIC-1 Linux Administrator 101-500 Exam FAQ, LPIC-1 Exam 101 Objectives, GNU and Unix Commands (Total Weight: 25)