Concept:Use the formula for the number of elements in the union of two sets.Explanation:The formula is n(A∪B)=n(A)+n(B)−n(A∩B).Substitute the given values: n(A)=8, n(B)=12, and n(A∩B)=3.So, n(A∪B)=8+12−3.This gives n(A∪B)=20−3=17.Answer:n(A∪B)=17, which is option B.