Formal:
* क्या हुआ? (kya hua?) - This is the most common and neutral way to ask "what happened?"
* क्या घटित हुआ? (kya ghatit hua?) - This is a slightly more formal way of asking.
Informal:
* क्या बात है? (kya baat hai?) - This is a more casual way of asking "what's up?" or "what's the matter?"
* क्या हुआ भाई/बहन? (kya hua bhai/bahan?) - This is a more familiar way of asking "what happened?" to someone you know well.
Other options:
* क्या हो गया? (kya ho gaya?) - This is another way of saying "what happened?" that is often used in a more informal setting.
* क्या बात है? (kya baat hai?) - This is a more casual way of asking "what's up?" or "what's the matter?"
The best way to choose the right phrase is to consider the context and the person you are talking to.
Example:
* If you are asking a stranger what happened, you would use "क्या हुआ?" (kya hua?).
* If you are asking a friend what happened, you could use "क्या बात है?" (kya baat hai?) or "क्या हुआ भाई/बहन?" (kya hua bhai/bahan?).
I hope this helps!