Which of the following options is used to gzip a file called filename?

The command used to gzip a file named "filename" is simply "gzip filename." This command compresses the specified file using the gzip (GNU zip) compression algorithm. When executed, it replaces the original file with a compressed version, giving the new file the same name but with a ".gz" extension added (e.g., filename.gz).

The other options either do not fulfill the requirement to compress the file or are not valid commands for gzip. For instance, using "-c" with gzip sends the compressed file to standard output rather than replacing the original file, which is not what the question asks. The "-x" option is not related to compression and typically is used for extraction with other utilities. Finally, "-z" is unnecessary because gzip by default applies compression when provided with a file name, making it redundant in this context.