Concept:The relationship is that each number in the set increases by 6.Explanation:For the given set (9,15,21):9+6=15 and 15+6=21.For (31,37,43):31+6=37 and 37+6=43.So the rule is: first number +6 = second number, second number +6 = third number.Now check the options:Option A: 32+6=38, but 38+6=44î€ =43.Option B: 45+6=51, but 51+6=57î€ =56.Option C: 65+6=71 and 71+6=77. This follows the rule.Option D: 23+6=29î€ =28.Answer:The correct answer is Option C: (65,71,77).