The correct answer is option 'B', which states that the difference between a shift-right register and a shift-left register is the direction of the shift. Let's understand this in detail:

Shift Registers:
Shift registers are sequential logic circuits that can store and shift binary data. They consist of a series of flip-flops connected in a chain, where each flip-flop holds a single bit of data. The input data is shifted through the flip-flops based on a clock signal.

Shift-Right Register:
In a shift-right register, the data is shifted towards the right. This means that the data moves from the leftmost flip-flop to the rightmost flip-flop. The input data enters the leftmost flip-flop, and upon each clock pulse, it is shifted to the right, with the rightmost bit being lost and a new bit entering the leftmost flip-flop. This results in a rightward movement of data.

Shift-Left Register:
In contrast, a shift-left register shifts the data towards the left. The input data enters the rightmost flip-flop, and upon each clock pulse, it is shifted to the left. The leftmost bit is lost, and a new bit enters the rightmost flip-flop. This leads to a leftward movement of data.

Propagation Delay:
Propagation delay refers to the time taken for a signal to propagate through a circuit. It is the time delay between the input change and the corresponding output change. Both shift-right and shift-left registers have propagation delays, but the direction of the shift does not affect this delay. Therefore, propagation delay is not the difference between the two types of registers.

Clock Input:
The clock input is a common component in shift registers. It controls the timing of the shift operation by triggering the flip-flops to change their states and shift the data. However, the clock input is not the difference between a shift-right and a shift-left register. Both types of registers use a clock input for their operation.

Conclusion:
In summary, the main difference between a shift-right register and a shift-left register is the direction of the shift. In a shift-right register, the data is shifted towards the right, while in a shift-left register, the data is shifted towards the left. The other factors such as propagation delay and clock input are common to both types of registers.