Which of the following gates outputs HIGH only when all inputs are LOW?

The gate that outputs HIGH only when all inputs are LOW is the NOR gate. A NOR gate is a combination of an OR gate followed by a NOT gate. It produces a HIGH output only when its inputs are all at a LOW state. In any other circumstance—where one or more inputs are HIGH—the output will be LOW.

To elaborate, the operation of the NOR gate can be defined by the truth table, where only when the inputs are (0, 0, ..., 0) does the output yield (1). For any input combination that includes at least one HIGH, the output is (0). This makes the NOR gate a fundamental component in digital logic especially for creating negative logic.

Other gates behave differently. An AND gate produces a HIGH output only when all inputs are HIGH. A NAND gate, which is the inverse of the AND gate, outputs LOW only when all inputs are HIGH. An XOR gate outputs HIGH only when an odd number of its inputs are HIGH. Understanding these characteristics highlights why the NOR gate is uniquely positioned to meet the specified condition of outputting HIGH exclusively with all LOW inputs.