I had _______ that I would always remember it.A.so a good experience B.such a good experienceC.so good an experience D.such good an experience,我知道B,可是C也对啊.说出C不对的地方啊

就是选B

so good an experience
so+形容词+ a/an+名词固定用法 是 太。。。。的一个。。。。了
本人英语专业研究生 你可以查查语法书

such…that作“如此…以致”解，连接一个表示结果的状语从句。与so…that意思相同，但用法不同。
so…that这一结构中，so后边可加形容词或副词，而such后边一定要用名词（这个名词前面可以带形容词，也可以不带）。

只需记住，后面带名词的话，一般优先考虑使用such。但如果such后边的名词前由many、much、few、little等用于形容量的形容词所修饰的话，则不用such而用so。
例如：
He had so little education that he was unfit for this job.
There were so many people in the street watching the fire that firefighters could not get close to the building.

个人认为B

如果用C,应该是 a so good experience