Logic Gates question from JEE Main 2020 (7 Jan, Shift 1)

Answer: (b)

1. A reversible gate lets the input be found uniquely from the output, so it needs a one-to-one input–output map.
2. Two-input NOR, OR and NAND give the same output for several input combinations, so the input cannot be recovered.
3. A NOR gate with both inputs joined is a NOT gate: $0 \to 1$, $1 \to 0$, which is one-to-one.
4. So the answer is (b).