Answer:
Dad is stirring the soup.
Explanation:
All the other three sentences uses the passive form.
The table is being set by the girls.
The lettuce is being cut by Mom.
The napkins were folded by the boys.
Up there I've underlined the passive tenses and bolded the main verbs. If that still doesn't help, the italic by should show that the one doing the actions are mentioned at the last by choice only, and not necessity. Which means those are passive forms because the actors are not important.