Final answer:
Among the options provided, the set of Philippine Presidents is a well-defined set because it refers to a specific and identifiable list of individuals, as opposed to subjective categories like happiness or goodness.
Step-by-step explanation:
The concept of a well-defined set in mathematics involves a set where there is a clear rule or property that determines membership. A well-defined set should allow for any object to be tested for membership and should not rely on subjective judgement.
The set of Philippine Presidents is a well-defined set because it refers to a specific list of individuals who have held the office of President in the Philippines. This can be clearly determined and does not involve personal opinion or subjective criteria. Unlike the set of happy people, good teachers, or enjoyable subjects, which rely on individual perspectives and feelings, the set of Philippine Presidents can be definitively known and documented.