In which operating region is the bipolar junction transistor (BJT) function "fully-ON" and works effectively as a switch?

Correct option is C

In the Saturation Region, the BJT is "fully-ON" because both the base-emitter and base-collector junctions are forward-biased. This condition allows maximum current to flow from the collector to the emitter, with a minimal voltage drop across them (acting like a closed switch). This makes the BJT an effective "closed switch" in digital and switching applications.