You are not graded on your ability to express yourself formally or informally, you are evaluated on your use and command of the English language, so it shouldn't matter, however, I would suggest using standard language free of jargons and slang.

They are looking for proper grammar usage and expanded vocabulary, so use words properly and follow grammar rules. Formality doesn't really matter, but I would stick to words you can find in the encyclopedia for both the written and spoken portions.