Final answer:
A 'normally open' switch refers to the switch being in the off position by default. It has high resistance when open, preventing current flow, but allows current to pass with minimal resistance when closed. The correct option is C.
Step-by-step explanation:
For a switch, normally open indicates that the switch is normally in an off position. When we say a switch is normally open, we mean that in its default state, without any external force or influence applied to it, there is no electrical connection being made. This is the standard terminology used in electrical engineering and circuit design.
In the context of a circuit diagram, such as in Figure 21.46, when a switch has extremely large resistance when open, no current can flow through it.
However, once the switch is closed, it will have nearly zero resistance and allows the current to flow through the circuit, activating any devices like light bulbs or motors that are connected in series with the switch. The correct option is C.