Answer- D (400)In this question, use Standard Error of propoions.The 95% confidence limits for a propoion are p +- 1.96 s.e.p, where s.e.p is the standard error of a propoion.p = sample propoion = propoion of diseased persons = 0.5 q = 1-p = 0.5 s.e.p .NI(p x q / n) Now here, the range of propoions with 95% confidence is mentioned as? 45-55%Hence, 2 s.e.p = 0.55-0.50 = 0.05 s.e.p = 0.0250.025= J(pxq/n)n = 400Thus, the sample size required will be 400.