I look forward __ your letter. 

I look forward __ your letter.  সঠিক উত্তর to receiving

If the TO is part of a phrasal verb or a verb + preposition combination: A phrasal verb is something like "look forward to", "confess to", etc. In this case, the "TO" is part of the verb itself and is followed by -ing. - I'm looking forward to meeting my friend again.
Bissoy MCQ