The school has a regulation ____ all the teachers and studen

C
同位语从句,
that = all the teachers and students must wear school uniform on Mondays.
同位语从句可以变成两个句子.
The school has a regulation.
The regulation is that all the teachers and students must wear school uniform on Mondays.
由于all the teachers and students must wear school本身就是一个句子,如果用which的话,你说which能代替哪个词呢?如果你找不到可以代替which的,那就用that.这就是最直接的方法.

引导同位语从句一般都是用that, which在同位语从句中的用法不多，指的是在一定的范围内，哪一个，一般充当定语，而且只用在某些名词的后面，常见的有 idea ：
I have no idea which picture is the most beautiful of all.
而且名词性从句中that只起引导作用，不在从句中做任何成分，而这里也不缺句子成分，所以用that...

C同位语从句