Sentence A is grammatically correct.
In Sentence B there is a tense mismatch. It should be either 'Before the rain stopped, they had reached home'(past report) or 'Before the rain stops, they will have reached home'(present possibility). The only other possibility is 'Before the rain stopped, they would reach home'(a habitual past action).
In Sentence C too there is a mismatch error. As a conditional or a possible future action it should be 'When you come to see me, we will go to Mumbai.
Sentence D should be 'I have written both to their branch office and their/the head office'.
Hence, option A is the correct answer.