Pick the appropriate preposition for the blank space: Burn the love letter ____ Ashes.

The right answer is: To 

To
- এটা মূলত: preposition.
- এছাড়াও এটা  part of the infinitive (the to-infinitive) হিসেবে ব্যবহৃত হয় । 

To as a preposition: destination or direction
We can use to as a preposition to indicate a destination or direction: যেমন:  The dog ran to us as soon as we arrived.

এছাড়াও as a preposition, to এর বিভিন্ন রকম অর্থ হতে পারে, যেমন:

- Receiver of an action. Example: She’s always writing letters to the local newspaper.

- Time: Example:  Her train arrives at quarter to five.

- Approximate numbers. Examples: There were forty to fifty people at the meeting.

Source: CLLIFS TOEFL