I can't remember how many days ago --- a fight broke out bet

这句不是强调句.而是宾语从句套主语从句.
因为how many days ago 以及之后的部分是remember的宾语从句.宾语从句应该是陈述句的形式,所以排除C,D
而宾语从句中,a fight broke out between them 是主语从句 ,引导词应该用that,不用when.it 做了a fight broke out between them的形式主语
还原成正常的语序应该是 it was (how many days ago) that a fight broke out between them.

强调句呀这是个

首先这是一个强调句做remeber的宾语的句式。下面我们把它单独抽离出来：
--------------------how many days ago____a fight broke out between them.从选项我们就可以看出出题者的意图是考察强调句的句式。我们先将它还原看是否符合强调句的要求。
It was {how many days ago} (when...