参考答案：D

解析：关系代词which替代的是synthetic materials，实际上要求选择的是介词。between指两者之间，而句子中是many kinds, of在与形容词最高级连用表示比较范围时，多用于all等确定的范围，如Of all the synthetic materials, plastics are the most common，而句中的many是个不确定的概念。in在表示比较范围时，后接的名词多有整体范围的含义，试比较：He is the best student in his class. He is the best student of the three. among“在……当中”，相当于…plastics are the most common among synthetic materials。