Make the following sentence passive: 

Who wrote this?

The correct passive form is: "By whom was this written?"

Explanation:

1. "This is written by who?" - This is not a grammatically correct passive construction. In English, we use "by whom" instead of "by who" when forming a passive sentence.

2. "By who was this written?" - This is incorrect because "who" should be replaced with "whom" when used in a passive construction. The correct form is "By whom was this written?"

3. "Whom was this written by?" - This is a correct passive construction. However, in formal English, it's more common to place the auxiliary verb before the subject, resulting in "By whom was this written?"

4. "By whom was this written?" - This is the correct and formal passive construction. It follows the standard English structure for passive sentences, where "by whom" is used.

In summary, the most appropriate and grammatically correct passive form is "By whom was this written?"