The answer 'A' is based on the feedback from our users and verification by our Toppers.Note: the explanation below is based on earlier answer option 'C' Ans is 'a i.e. Measles Note-A few points about measlesMeasles virus has only one serotypeIt can't survive outside the human body for any length of timeOnly source of infection is a case of measles. "CARRIERS are not known to Occur".Measles is highly infectious during the prodromal period and at the time of appearance of rash.The communicability is approx 4 days before to 5 days after the appearance of the rash* Carriers - A carrier is defined as an infected person or animal that harbors a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection for others.Carriers may be classified as:Incubatory carriersConvalescent carriersHealthy carriersIncubatory carriers -Incubatory carriers are those who shed the infectious agent during the incubation period of the disease.eg. Measles, Mumps, Polio, Pertussis, Influenza, Diptheria, Hepatitis B.Convalescent carriers -Those carriers who continue to shed the disease agent during the period of convalescenceeg. Typhoid, Dysentery, Cholera, Diptheria, Whooping CoughHealthy carriers -Healthy carriers are those who have developed the carrier state without suffering from the overt diseases. Healthy carriers emerge from subclinical caseseg. Polio, Cholera, Meningococcal Meningitis, Diptheria, Typhoid.Carriers can also be classified according to duration:Temporary carriersChronic carriersTemporary carriers -are those who shed the infectious agents for a short period of time.Chronic carriers -is one who excretes the infectious agents for an indefinite period.eg. Typhoid, Hepatitis, Dysentery, Malaria, Gonorrhea, Diptheria, Cerebrospinal meningitis.What are pseudo carriers? -Pseudo carriers are carriers of avirulent organisms (organisms which are not able to cause any disease)