Explanation:

- Option 1: why

- This is the correct choice.

- The phrase "the reason why" is commonly used in English to explain the cause of a situation.

- It's a standard expression that fits well in the sentence to indicate the cause or reason.

- Correct Answer: why

- Option 2: since

- "Since" implies a relationship over time or reasoning but doesn't fit the context here clearly.

- It's less standard to say "the reason since," making it awkward.

- Option 3: because

- "Because" explains reasons, but "the reason because" is redundant.

- Typically, we use "because" without pairing it this way in a sentence.

- Option 4: so

- "So" serves to express results or purposes rather than being tied to expressing reasons directly.

- "The reason so" would not be a clear expression.