Discussion
For the given logic family, if all the inputs (A, B) are high, then output Z will be:
Answer: A
The circuit is a TTL NAND gate with a totem pole output: a multi-emitter input transistor T1, a phase splitter T2, and the output pair T3 (with the diode D) and T4.
When all the inputs A and B are high, the base collector junction of T1 conducts, so current from the supply through the 4 kilohm resistor flows into the base of T2, which turns on and in turn drives T4 into saturation. The emitter voltage of T2 turns T4 on while the voltage at the collector of T2 falls too low to keep T3 and the diode conducting, so T3 is off. The output Z is pulled to ground through the saturated T4, so Z is low, logic 0.
That is the NAND gate action: the output is low only when all the inputs are high. The answer is 0, option A.
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