Concept:The union of two sets contains all elements from both sets.Explanation:Given X∪{a,b}={a,b,c,p,r}.Elements c, p, and r appear in the union but not in {a,b}, so they must belong to X.Elements a and b are already supplied by {a,b}, so X does not need to contain them.Thus the smallest possible X is {c,p,r}.Answer:{c,p,r}.