or
or
We use the Present Simple or Indefinite to talk about general facts that are always true and permanent about the world.
So the sentence should be Present Simple.
Here, smoking is a Gerund and used as an Uncountable Noun. Unountable Nouns are always singular in form.
As per the rules of formation a Present Simple sentence, we use the base form of the verb and add -s or -es for the third person singular.
So the answer is Affects.
