Question

Which phrase best completes this rule? _____________ doit jeter ses ordures dans la rue.?

Answer 1

Which phrase best complete this rule ?

Personne ne doit jeter ses ordures dans la rue

= Nobody must throw its trash in the street

nobody = personne, but in French , you still have to had the negation "ne", otherwise it's incorrect. 

Hope I helped !

Answer 2

Which phrase best completes this rule?
Personne ne doit jeter ses ordures dans la rue.