What is the function of an SPDT switch?

SPDT stands for Single-Pole, Double-Throw. In electrical terminology, the 'pole' refers to the number of separate input circuits controlled by the switch, while the 'throw' refers to the number of distinct positions or connections the pole can make. Because it is a 'single-pole' switch, it controls one single input circuit. Because it is a 'double-throw' switch, it can route that single input to either one of two different output circuits. The distractors describe different switch types. For example, 'A single circuit is opened or closed' describes an SPST (Single-Pole, Single-Throw) switch, and switching two separate circuits would require a Double-Pole (like a DPDT) switch.