_me the most is that these butterflies return to the very same tree in Mexico that their great great-grandparents used for the winter before.A. It amazedB. Which amazedC. That amazedD. What amazed

步骤 1：分析句子结构
句子的主干是“_ me the most is that these butterflies return to the very same tree in Mexico that their great-great-grandparents used for the winter before.”，其中“_ me the most”是主语，后面是主语从句。
步骤 2：确定主语从句的引导词
主语从句需要一个引导词，这个引导词需要在从句中充当主语。在选项中，只有what可以引导主语从句，并且在从句中充当主语。
步骤 3：选择正确的选项
根据句子结构和引导词的要求，选择D选项“what amazed”作为主语，符合句子的语法和语义要求。