Concept:Cholera is caused by bacteria (Vibrio cholerae) that spread through food and water contaminated with human faeces.Solution:Improving sanitation (proper toilets, safe disposal of refuse, clean drinking water, washing hands and covering food) stops the germs from reaching food and water, so it prevents outbreaks.Supplying drugs and providing clinics treat people who are already sick; improving nutrition does not stop the germs spreading.Answer:Option D (improving sanitation)