I am trying to understand ____ that makes him late for work again.A. what it isB. W why it is c why it does

步骤 1：分析句子结构
句子的主干是 "I am trying to understand ____ that makes him late for work again."，其中 "that makes him late for work again" 是一个定语从句，修饰先行词。
步骤 2：确定先行词
先行词是 "____"，需要填入一个名词性从句，作为 "understand" 的宾语。
步骤 3：选择合适的选项
选项 A "what it is" 是一个名词性从句，可以作为 "understand" 的宾语，符合语法要求。
选项 B "why it is" 和选项 C "why it does" 都是疑问词引导的从句，但它们都是疑问句，不符合句子结构。