Answer:
A. Slang or filler words
Step-by-step explanation:
Formal writing is the form of writing and speaking that we use in situations that demand a degree of seriousness. This type of writing is based on the correct use of grammatical norms as well as the good pronunciation of words. For this reason, it is not correct, in this type of writing, the use of slang or words of filling, because it would take the formal and revocated pattern that this type of writing requires.