答案：C

题目分析：考查定语从句：句意为：他写了一封信。在信中他解释了事故中发生的一切。“he explains what had happened in the accident.”部分为一定语从句，修饰先行词a letter。而且该从句本身主(he )--谓(explains )--宾(what had happened in the accident)成分齐全，缺少的是地点状语。所以用关系副词where.选C。