Concept:Probability is the ratio of favourable outcomes to total possible outcomes.Explanation:When a die is tossed once, the sample space is S={1,2,3,4,5,6}.Thus, total number of possible outcomes =6.The prime numbers in this set are 2,3,5.So, number of favourable outcomes =3.Required probability =total outcomesfavourable outcomes=63=21.Answer:21Option A.