Final answer:
The set of mobile games for the year 2020 may or may not be well-defined depending on the criteria used to define it. Hence the correct answer is option D
Step-by-step explanation:
The answer to the question is D. Depends on the criteria used.
A set is well-defined if its elements can be determined without any ambiguity or uncertainty. While the set of mobile games for the year 2020 may seem well-defined at first, it actually depends on the criteria used to define what constitutes a mobile game. If the criteria are clear and specific, such as considering only games released on mobile platforms during the year 2020, then the set can be well-defined. However, if the criteria are vague or open to interpretation, then the set may not be well-defined.