Final answer:
The NAND function in digital logic produces a high output when any input is low, which is the opposite behavior of an AND gate.
Step-by-step explanation:
The unique output behavior of the NAND function in digital logic is that it produces a high output when not all inputs are high. Specifically, it will produce a high output when one or more inputs are low. Conversely, the only time it produces a low output is when all inputs are high. Therefore, the correct answer is d) It produces a high output when any input is low.
The NAND gate works inversely to an AND gate which would give a high output only when all its inputs are high. NAND can be considered as an AND gate followed by a NOT gate. Therefore, the NAND gate output is the complement of the AND gate's output.