What is the best reason a formal tone and language should be used in professional electronic communications? informal communications are more easily misinterpreted. informal communications provide a written record. the use of formal tone and language is required by most businesses. using formal tone and language makes you sound smarter.

The best answer is that "The use of formal tone and language is required by most businesses". 
The fact that most communication in business these days takes place electronically does not change the expectation of formality in professional communications. The general rule in business is to be polite, eloquent, and direct in your communication, and thus a formal tone and language is best.