Answer:
No tire basura en el parque.
Explanation:
The formal style of writting in spanish is when you talk to people in "Usted" form and that is accomplished by speaking in third person to the person, you never speak in second person to someone that you are approaching formally.
So the only one from the options that is not speaking directly in "You" form rather and a "He/she" form or third person form is "No tire basura en la calle", so that would be the formal command written properly.
