The most appropriate topic for Helena's report on a communicable disease would be hepatitis C. Hepatitis C is a viral infection that is transmitted from person to person, primarily through blood-to-blood contact, making it a communicable disease.
In contrast, asthma and diabetes are non-communicable diseases, and cystic fibrosis is a genetic disorder rather than an infectious disease. Therefore, focusing on hepatitis C would align best with the criteria of a communicable disease.