A: ________ is responsible for all this confusion? B: It may be Jack. ?

In the dialogue presented, the question posed by speaker A is seeking to identify the person culpable for a specific situation characterized by confusion. The correct word to fill in the blank is "Who" because it is used to inquire about the identity of a person.

Let’s analyze the options:

- A) How: This word questions the manner or method, which is not fitting in this context as the query is not about the approach to the confusion.
- B) Why: This option seeks the reason behind an event. However, the context calls for identifying a person, not seeking an explanation for the confusion.
- C) Who: This interrogative pronoun directly targets a person or people. In this case, it correctly addresses the confusion by asking for a specific individual's identity responsible for it.
- D) Whom: Although "whom" can refer to people, it is typically used in objective cases, which isn’t suitable in this active questioning format.

Thus, in this scenario where identification is key, "Who" is the most appropriate and grammatically correct choice, making it the best fit for the blank.