What is used to indicate a failure when two identical circuits operate in parallel?

The correct answer is the XOR gate. In digital logic, an XOR (exclusive OR) gate is used to detect differences between two inputs. When two identical circuits operate in parallel, the expected behavior is that both circuits will produce the same output when functioning correctly. If a failure occurs in one of the circuits, the outputs will differ.

The XOR gate evaluates its two inputs and outputs a high signal (1) when the inputs are different (one is high and the other is low), which effectively indicates a failure in one of the identical circuits. In this scenario, if each parallel circuit provided the same expected result, a difference in output would signify a failure in one of the circuits, which the XOR gate can successfully indicate.

Therefore, in the context of monitoring parallel circuit performance, the XOR gate is particularly useful for identifying discrepancies and potential failures efficiently.