The best way to see if there is a nonrestrictive clause is by taking out the information between the two commas. If the sentence still makes sense, it is nonrestrictive.
Mrs. Smith, who’s son attends my school, is my favorite teacher.
If we remove what’s in between the commas, it’ll say Mrs. Smith is my favorite teacher. This is the correct answer.
A water filter to help the underprivileged who can’t access clean water