I was hoping for some options for answering the question. Since there are no options given so i have to answer the question based on my knowledge. The best explanation why including a salad bar on a school lunch menu would be an advantage for students on restricted diets is that the students can also go for any kind of healthy food.
It would give students more options when choosing what to eat at lunch and will encourage more students to eat lunch at school. Those that are on restricted diets will actually have something to eat at school which is very important, all dietary needs should be catered for in all schools.
