The correct choice is:
**when the accident happened**
Explanation:
1. **at which the accident happened**: This phrase is grammatically correct, but it sounds more formal and is not commonly used in everyday language. It may be more suitable for formal writing or specific contexts.
2. **the accident happened**: This phrase is grammatically correct, but it doesn't provide the temporal information (time) that the original sentence seems to be seeking.
3. **when the accident happened**: This is the most appropriate choice. "When" is a temporal adverb that directly addresses the time of the accident, making the sentence clear and concise.
4. **when the accident had happened**: While grammatically correct, the past perfect tense ("had happened") is generally used when you are discussing two past events and want to indicate which one occurred first. In this context, the simple past tense ("when the accident happened") is more straightforward and commonly used.
