Which command is used to change the ownership of a file?

The command used to change the ownership of a file is "chown." This command allows a user with the appropriate permissions (typically the superuser or file owner) to change the owner and optionally the group associated with a file or directory. For example, the command chown username:groupname filename sets the specified user and group as the new owners of the file.

In contrast, "chmod" is used to change the permissions of files or directories, not ownership. The other two options, "setown" and "ownerchange," are not valid commands in Linux for changing ownership. Thus, the specificity and functionality of "chown" are what make it the correct answer for altering file ownership.