Plain speaking or intelligibility is needed in a business letter.

a. True

b. False

It's true. But you should refer to a tone of writing and don't use slang and other elements of common spoken laguage so stay sharp and polite.

This is true. Intelligibility means that something is able to be understood. In a business letter, you want to make sure you use simple, uncomplicated language so that all parties that will read that letter will understand its meaning. Avoid lofty or confusing words and phrases in business letters.