参考答案：B

解析：这个句子结构比较复杂：how economic and social and political changes have played their part是show的宾语从句；so that…the postage stamp…has always been a faithful mirror of the times是结果状语从句；in which millions of people…find pleasure, knowledge and even profit是定语从句，修饰postage stamps; above all, correctly interpreted, young and old都是插入语。interpret的逻辑主语是postage stamp，与之是受动关系，所以用过去分词。