I believe the answer is: providing an additional choice for students on restricted diets.
Vegetarian menu tend to provide an option for various source of vitamin and minerals that might not exist on a typical high school lunch menu. It also might help the students with higher bmi index and higher cholesterol level who need to restrict their food intake before putting themselves on health risks.
What would happen if a vegetarian hot-food item was not added to the lunch menu? People who don't eat non-vegetarian food wouldn't be able to eat. :) The first option doesn't make sense. The second option is not the most useful advantage. The fourth option is also not the answer.
