Index case is the -

Ans. is 'd' i.e., First case defected by investigator [Ref: Park 22nd/e d. 92 & 21st/e p. 9/1o Primary caseo Index case--First case of communicable disease introduced into the population.First case to come to attention of the investigator.o It is not always necessary that primary case will be index case : -When the first case noticed by the investigator is also the first case of the population - Primary case and index case are the same.When the first case nociced by the investigator is not the first case of the population (primary case) - Index case and primary case are different.o Secondary cases - Cases those developing from contact with primary case.