Ans. is 'c' i.e., Identify new converters to tuberculin test Incidence of infectionIt is the percentage of population under study who will be newly infected by mycobacterium tuberculosis among the non-infected of the preceding survey during the course of one year.Persons newly infected with mycobacterium T.B. can be easily detected by new convertors to tuberculin test (remember positive tuberculin test indicates that the person is infected with M. Tuberculosis) it does not prove that the person is suffering from disease.Therefore the incidence of infection is also called "Tuberculin conversion index".This parameter is considered one of the best indicators for evaluating the tuberculosis problem and its trend.