The correct answer is C, Monitor.
Monitor has multiple meanings. Taking it as a verb, Monitor means observing or checking the progress of something over a period of time. Monitor is something that keeps an eye or watch something.
Taking it as a noun, a Monitor is a device that is used to display graphics. Monitors are connected to some systems and all activities being carried upon by the system are displayed through monitors. They are commonly used with the Computers, laptops, heart systems, etc.
In some countries of the world, the word monitor is also used for a person in a classroom who keeps record of the activities of the students.
So Monitor is the word with multiple meanings.