communication methods

You have a team spread across five countries. What is the best communication method to follow?

A)Formal Verbal

B)Informal Verbal

C)Informal Written

D)Formal Written

I have answered it as Informal Written. Whereas the answer is given as Formal Written. Can someone explain as to when to use formal written (in addition to correspondence with contractor) and to when to use informal written?

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I think D is correct, for permanent record as evidence to all, all the time.