[Grammar] By whom, who by or by who?

[Fear not only believe]

We were writing an English exam and, among other things, there was a task in which we had to write the correct questions to given sentences.
One of them was: John was told not to ask silly questions. Who _______?

Now, I put here by (which was correct), but later I had a bit of a discussion with my teacher about whether Who by or By whom is more common.
I think it is By whom, so what do you say? Who by?
By whom?
By who?
Are they all even gramatically correct?

 

[Tdol]

By whom is correct, but preposition + whom is mostly found in formal language, especially writing. Who by sounds more natural to me for conversation and is more common.

 

[Barb_D]

I think "Who told him that?" sounds better than "Who by?"

 

[Fear not only believe]

I think "Who told him that?" sounds better than "Who by?"

Yes, but Who _____? was already given.

 

[Barb_D]

And you could use only one word?

 

[Fear not only believe]

And you could use only one word?

Yes

 

[MikeNewYork]

Then it is a very silly question. I might have answered "says".