We have three options for the C4 on the plane:
1. Widmore 2. Ben, Richard, and Miles 3. MIB
It does not make sense for either of the last two to have done it. Ben and company would have blown up the plane right away instead of setting a trap. Also, from what we were shown, MIB did not place it, as he had to find it on the plane. It does seem he expected it to be there, as he did take the watch off the guy before finding the C4, but it does not make sense for him to have placed it only to take it away.
Now, Widmore is the only logical choice. Granted, there has been one big objection from some people: Widmore would have killed the candidates and others with MIB. However, this is answered by Widmore's actions earlier in the episode. He never intended for anyone to be with MIB when MIB tries to leave. Or at least, anyone considered important to him (the people on his list.) Widmore has been shown to not be the nicest person there is. Is it hard to believe he would be willing to sacrifice a couple people? (As long as they weren't candidates) And where were the candidates? He locked them in a cage so they COULDN'T go with MIB on the plane. Widmore was protecting the candidates by locking them in the cage, not holding them captive. He didn't want them to go the plane that he had set up as a trap.