A formal tone is most appropriate to use in-?

A. Emails

B. Essays

C. Conversations

D. Friendly letters

Essays because this is a formal form of writing. You are not expressing yourself here but you are trying to provide facts, to inform, to argue, etc. in which intimate level of register is note used. You may use a formal tone in conversation too if the person you are talking too is of higher authority because you have to pay respect to them for being superior.