Answer:
The answer is conoces.
Explanation:
It's "conoces" because since the sentence is a question and not a statement the only word that would grammatically work with the sentence is that word.
No digas eso a tu hermano.
How would you say "serve the food" to someone you don't know very well?
Answer : D.) Sirva la comida
Explanation : "Usted" is the formal subject when you are talking to someone you do not know very well, such as a waiter in a restaurant. Then the imperative mode must be conjugated in the third person. Usted sirva la comida.
Hope this helps!