Concept:A finite set has a fixed countable number of elements, while an infinite set cannot be counted completely.Explanation:The set {3,6,9,12} contains exactly 4 elements, so its elements can be counted. Therefore, {3,6,9,12} is a finite set. Options A and B use “…” at the end, meaning the pattern continues endlessly, so they are infinite sets. Option C also begins with “…” , showing that its list extends without a clear beginning or boundary. Only the set in option D has a definite, countable number of elements.Answer:D {3,6,9,12}