In Unix-like operating systems, chmod is the command and system call which may change the access permissions to file system objects (files and directories). It may also alter special mode flags. The request is filtered by the umask. The name is an abbreviation of change mode.

Answer this question: how to change permission to a file in unix? Let's think fast. Stream of consciousness:
The important part for answering this question is: In Unix-like operating systems, chmod is the command and system call which may change the access permissions to file system objects (files and directories). So the final answer is chmod.