What answer is correct to who should we send the letter of commendation or to whom should we send the letter of commendation?

Answer:
To whom should we send the letter of commendation is correct.

"Whom" is the object of the preposition "to" and so should be in the objective case.
First answer by Hammernose. Last edit by Hammernose. Contributor trust: 30 [recommend contributor recommended]. Question popularity: 1 [recommend question].