The truth table for an S-R flip-flop is used to represent the behavior of the flip-flop based on the inputs provided. It includes all possible combinations of inputs and their corresponding outputs. In this case, we are specifically asked about the number of valid entries in the truth table for an S-R flip-flop.

An S-R flip-flop has two inputs: the S (set) input and the R (reset) input. These inputs determine the state of the flip-flop. The S input sets the flip-flop to the state 1, while the R input resets it to the state 0. The outputs of an S-R flip-flop are the Q (output) and the Q' (complement of the output).

To determine the number of valid entries in the truth table, we need to consider all possible combinations of inputs. Since there are two inputs (S and R), each with two possible values (0 or 1), there are a total of 2^2 = 4 possible combinations.

Now, let's analyze each combination:

1. S = 0, R = 0:
- In this case, both inputs are 0, which means there is no change in the state of the flip-flop. It remains in its current state.
- The Q and Q' outputs will depend on the previous state of the flip-flop, which makes this combination invalid.

2. S = 0, R = 1:
- When the reset input R is set to 1, it overrides the set input S.
- The flip-flop is forced into the reset state (Q = 0, Q' = 1).
- This combination is valid.

3. S = 1, R = 0:
- When the set input S is set to 1, it overrides the reset input R.
- The flip-flop is forced into the set state (Q = 1, Q' = 0).
- This combination is valid.

4. S = 1, R = 1:
- In this case, both inputs are 1, which creates a race condition.
- The behavior of the flip-flop is undefined in this situation, as it depends on the specific implementation of the flip-flop.
- This combination is invalid.

Based on the analysis above, we can see that there are three valid entries in the truth table for an S-R flip-flop (combinations 2, 3, and 4). Therefore, the correct answer is option 'D': 3.