Well-Defined Sets

In elementary set theory, a set is a well-defined collection whose membership can be decided objectively and unambiguously. For option A, both conditions are precise: a number must be divisible by 5 with no remainder, and it must be less than 50. Therefore, the members can be listed as 5, 10, 15, 20, 25, 30, 35, 40, and 45, assuming positive natural-number multiples in this school-level context. The words “interesting,” “easy,” and “good” in options B, C, and D depend on personal judgement, so different people may include different objects. Hence only option A gives a clearly defined set.