What is the best interpretation of Mark Twain's famous admonition to not "use a five-dollar word when a fifty-cent word will do"?

A. Use slang and idioms whenever possible in your writing and conversations.
B. Always use the longest and most complex words to explain your thoughts.
C. When talking about money, use bills instead of coins.
D. Pick words that will allow you both to express yourself and make yourself understood.
E. None of these answers are correct.