In the circuit shown in the Figure, A and B are logic inputs and Y is the logic output. Which of the following logic operations is realized by the circuit?

The correct option is NOR.

By analyzing the provided circuit diagram, we can identify that it represents a classic Complementary Metal-Oxide-Semiconductor (CMOS) logic gate with inputs labeled A and B, a power supply voltage labeled +V_DD at the top, a ground connection at the bottom, and an output terminal labeled Y. The circuit consists of two networks: a pull-up network (PUN) at the top and a pull-down network (PDN) at the bottom.

1. Pull-up Network (PUN):
The upper part of the circuit contains two PMOS transistors (indicated by the inward-pointing arrows on the gates) connected in series between the supply line +V_DD and the output Y. The gates of these PMOS transistors are controlled by inputs A and B respectively.
- A PMOS transistor is active (ON) when its gate input is LOW (logic 0).
- Since the two PMOS transistors are connected in series, both must be ON simultaneously to create a conducting path from +V_DD to the output Y.
- Therefore, the pull-up network only pulls the output Y to a HIGH state (logic 1) when:
$A=0$ and $B=0$

2. Pull-down Network (PDN):
The lower part of the circuit consists of two NMOS transistors (indicated by the lack of bubbles or inward arrows at the gates) connected in parallel between the output Y and the ground.
- An NMOS transistor is active (ON) when its gate input is HIGH (logic 1).
- Since these two transistors are in parallel, the output Y will be connected to ground (logic 0) if either NMOS transistor or both are turned ON.
- Therefore, the pull-down network pulls the output Y to a LOW state (logic 0) if:
$A=1$ or $B=1$ (or both)

3. Truth Table Analysis:
Let us evaluate the output Y for all possible logic combinations of the inputs A and B:
• When A = 0 and B = 0: Both PMOS transistors are ON, and both NMOS transistors are OFF. The output Y is pulled up to +V_DD (logic 1).
• When A = 0 and B = 1: The PMOS controlled by B is OFF, breaking the pull-up path. The NMOS controlled by B is ON, pulling Y to ground (logic 0).
• When A = 1 and B = 0: The PMOS controlled by A is OFF, breaking the pull-up path. The NMOS controlled by A is ON, pulling Y to ground (logic 0).
• When A = 1 and B = 1: Both PMOS transistors are OFF, and both NMOS transistors are ON. The output Y is pulled to ground (logic 0).

This behavior corresponds exactly to the truth table of a 2-input NOR gate. The logic expression for the output is:
$Y=\overline{A+B}$
Thus, the logic operation realized by the circuit is indeed NOR.