Formal:
* কেন (ken) - This is the most common and versatile way to say "why".
* কিসের জন্য (kiser jonno) - This translates to "for what reason" and is more formal.
Informal:
* কি জন্য (ki jonno) - This is a shorter, more informal version of "kiser jonno".
* কারণ (karan) - This literally means "reason" but can be used in place of "why" in certain contexts.
Examples:
* তুমি কেন এখানে এলে? (Tumi ken ekhane ele?) - Why did you come here?
* কিসের জন্য তুমি এত রাগ করছো? (Kiser jonno tumi eto raag korcho?) - Why are you so angry?
* কি জন্য তুমি দেরি করেছো? (Ki jonno tumi deri korecho?) - Why were you late?
* তুমি এটা করলে কারণ? (Tumi eta korle karan?) - Why did you do this?
The best option for you will depend on the specific situation and the level of formality you wish to convey.