Which gate means "not and"?

The phrase "not and" refers specifically to the NAND gate. In digital logic, a NAND gate produces an output that is false only when all its inputs are true. This means that it inverts the output of an AND gate. Thus, when you say "not and," you are essentially describing the function of a NAND gate, which negates the operation of the AND gate.

In contrast, the other choices represent different logical operations. The NOR gate produces a true output only when all inputs are false, making it a "not or" operation. The XOR gate outputs true when the inputs are different, which doesn't relate to the "not and" concept. Lastly, the AND gate itself outputs true only when all inputs are true, without any negation involved. So, the designation of "not and" aligns solely with the NAND gate's function.