What reverses the voltage state from that of an AND gate in a NAND gate?

The NAND gate functions by first performing the operation of an AND gate and then applying a negation to that output. The output of an AND gate is HIGH only when both of its inputs are HIGH. By adding a NOT gate after the AND operation, the NAND gate produces an output that is the opposite of the AND gate's output. Therefore, if the AND gate outputs a HIGH state (1), the NOT gate changes it to a LOW state (0), and vice versa. This inversion is what characterizes the NAND gate, making it critical in digital logic design as it combines the functionality of an AND gate with the reversal of its output through a NOT gate.

The other options do not offer the negation required to produce the NAND effect following the AND operation. An OR gate does not perform the necessary AND operation; similarly, an XOR gate does not strictly provide the negation of an AND operation. Consequently, the presence of the NOT gate after the AND gate is what enables the transition to a NAND gate.