The S-R flip-flop is a fundamental building block in digital electronics, commonly used to store and manipulate binary information. It has two inputs, S (set) and R (reset), and two outputs, Q (output) and Q' (complement of output). The truth table for an S-R flip-flop lists the possible combinations of inputs and their corresponding outputs.

To understand the number of valid entries in the truth table, let's analyze the behavior of the S-R flip-flop. The flip-flop has two stable states: set and reset. When the set input (S) is high and the reset input (R) is low, the flip-flop is set and the output Q is high. Conversely, when the reset input (R) is high and the set input (S) is low, the flip-flop is reset and the output Q is low. When both inputs are high or both inputs are low, the flip-flop enters an undefined state, where the outputs Q and Q' can be unpredictable.

Now, let's construct the truth table for the S-R flip-flop:

| S | R | Q | Q' |
|---|---|---|----|
| 0 | 0 | ? | ? |
| 0 | 1 | 0 | 1 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | ? | ? |

- Undefined entries:
- When both S and R inputs are low (0), the flip-flop enters an undefined state. This is because the behavior of the flip-flop is not well-defined when both inputs are low. The outputs Q and Q' can be unpredictable, and thus, these entries are marked as undefined in the truth table.

- Valid entries:
- When the S and R inputs are such that one is high (1) and the other is low (0), the flip-flop is in a well-defined state. These are the valid entries in the truth table, as they represent the expected behavior of the flip-flop.
- In these valid entries, the output Q is determined by the set (S) and reset (R) inputs. For example, when S is high and R is low, the flip-flop is set, and thus, the output Q is high (1).

Therefore, the truth table for an S-R flip-flop has 3 valid entries, corresponding to the combinations of inputs that result in a well-defined state. These entries are:

| S | R | Q | Q' |
|---|---|---|----|
| 0 | 1 | 0 | 1 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | ? | ? |

Hence, the correct answer is option 'C' - 3 valid entries.