Well-Defined and Ill-Defined Collections

The governing idea is that a set must be a well-defined collection. There must be an objective rule that allows us to decide whether every proposed object belongs to the collection. In option C, the word “tasty” expresses personal preference rather than a measurable, universally fixed condition. One person may consider a drink tasty while another may not, so membership is uncertain and the collection is not well-defined. Option A can be determined by the condition n < 12 for natural numbers; option B is identified from the official roll list; and option D consists of the seven specifically named weekdays. These three collections have definite membership, so option C is the only unsuitable one.