The correct answer is A. Full adder.
A full adder is a logic circuit that adds two binary digits and a carry bit. It has three inputs and two outputs. The inputs are the two binary digits to be added and the carry bit from the previous stage. The outputs are the sum of the two binary digits and the carry bit for the next stage.
A half adder is a logic circuit that adds two binary digits. It has two inputs and one output. The inputs are the two binary digits to be added. The output is the sum of the two binary digits.
A buffer is a logic circuit that isolates two parts of a circuit from each other. It has one input and one output. The input is the signal from the first part of the circuit. The output is the same signal as the input, but it is isolated from the first part of the circuit.
A register is a logic circuit that stores data. It has one or more inputs and one or more outputs. The inputs are the data to be stored. The outputs are the data that is stored.
E is not the correct answer because it is not a logic circuit that can add two binary digits.