Trusted Computing Base (TCB) is a system which is equipped with all the minimum hardware and software required for the security purposes of the operating system. When TCB is in place the security of the system cannot be breached. Generally, the TCB is kept as a separate entity from the operating system.
Reference monitor is a part of the Trusted Computing Base. It controls all the security decisions like access control. It controls on which user or processes can read or write on which of the files. A system call must go through the reference monitor for security validation.
In the below given figure you can see reference monitor displayed as a part of the Trusted Computing Base: