What are the two stable operating conditions of a silicon controlled rectifier (SCR)?

A Silicon Controlled Rectifier (SCR) is essentially an electronic switch. As a switch, its fundamental purpose is either to block current or allow it to pass, which means its two stable states are non-conducting (off) and conducting (on). Once the SCR is triggered into the conducting state by its gate, it remains in that state until the current flowing through it drops below a specific holding threshold. When the current drops sufficiently, it snaps back to the stable non-conducting state. Other options represent irrelevant or impossible states for this component. For example, an SCR is a unidirectional device, meaning it cannot stably conduct in reverse (unlike a TRIAC). The terms NPN and PNP describe transistor construction types, not operating states.