Benjamin is a 6-year-old boy who is seen in the paediatric emergency department. He has been vomiting and has had diarrhoea for 3 days. His stool is watery and foul smelling but has no blood in it. He has not been out of the UK since he was born. Examination reveals mild dehydration but is otherwise normal. What is the most likely organism that has caused his symptoms? Select one answer only.