What makes NAND gate universal in digital logic?

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NAND is universal because a single type of gate can realize any boolean function when you combine multiple gates. The key is that NAND can create the basic operations needed to build any circuit: negation and conjunction. You can get NOT by tying both inputs together, so NOT A equals NAND(A, A). You can build AND by taking the NAND of A and B and then negating that result with another NAND: A AND B equals NAND(NAND(A,B), NAND(A,B)). You can build OR by negating the inputs first and then NANDing them: OR(A,B) equals NAND(NAND(A,A), NAND(B,B)). Since NOT, AND, and OR can all be implemented with NAND gates alone, any logic function can be constructed with just NANDs. The other options describe either a different gate’s behavior or claim you can’t obtain NOT, which isn’t true, since a NAND gate alone can produce NOT and the other basic operations through proper wiring.