A D flip-flop can be constructed from an S-R flip-flop.

Explanation:
To understand why a D flip-flop can be constructed from an S-R flip-flop, let's first understand the basic working of both flip-flops.

S-R Flip-Flop:
An S-R (Set-Reset) flip-flop is a basic flip-flop that has two inputs, S (Set) and R (Reset), and two outputs, Q (output) and Q' (complement of output). It operates based on the logic levels of the inputs as follows:

- When S = 1 and R = 0, the flip-flop sets and Q = 1.
- When S = 0 and R = 1, the flip-flop resets and Q = 0.
- When both S and R are 0 or both S and R are 1, the flip-flop holds its previous state.

D Flip-Flop:
A D flip-flop, also known as a data flip-flop, has a single input, D (data), and two outputs, Q and Q'. It operates based on the logic level of the input as follows:

- When D = 1, the flip-flop stores the value 1 and Q = 1.
- When D = 0, the flip-flop stores the value 0 and Q = 0.

Construction of D Flip-Flop from S-R Flip-Flop:
A D flip-flop can be constructed from an S-R flip-flop by connecting the inputs of the S-R flip-flop in a specific way. Here's how it can be done:

1. Connect the D input of the D flip-flop to both the Set (S) and Reset (R) inputs of the S-R flip-flop.
- This ensures that the S-R flip-flop behaves like a D flip-flop.

2. Connect the complement of the Q output of the S-R flip-flop to its Reset (R) input.
- This ensures that the S-R flip-flop resets when Q' = 1, which is the complement of the Q output of the D flip-flop.

3. Connect the Q output of the S-R flip-flop to the input of an inverter.
- This inverter generates the complement of the Q output of the S-R flip-flop, which becomes the Q' output of the D flip-flop.

By following these connections, we effectively convert the S-R flip-flop into a D flip-flop. The S-R flip-flop now behaves exactly like a D flip-flop, where the input D controls the state of the flip-flop and the outputs Q and Q' represent the stored value and its complement, respectively.