---she couldn't understand was -A---fewer and fewer students showed interest in her lessons
A what why
B that what
C what because
D why that
答案解析：答案A ：
【What】she couldn't understand was 【why】fewer and fewer students showed interest in her lessons
前面是连接代词what引导的主语从句,后面由连接副词why 引导表语从句
中文：她无法理解的是为什么越来越少学生表现得对她上课不感兴趣