Explanation:

- Original Statement: "If they will get married, they will probably settle in Mumbai."

- Option 1: "If they get married" – This is correct. In conditional sentences, the present simple tense ("get") is used to refer to the future, making it the right choice.

- Option 2: "No improvement" – Not correct. The original sentence's use of "will" in the condition clause is not standard in English.

- Option 3: "If they had got married" – This implies a hypothetical past situation, not suitable for the intended meaning about the future.

- Option 4: "If they will be getting married" – This is not standard for conditional statements, which use the present simple for future situations.