Question

This is the reason ____I was telling you about it.

Answer 1

你好。比较一下，你就会了：
1 This is the reason WHY I was telling you about it.--------後面主语谓语宾语都全了，就是没有状语（原因状语），你就选用why 或者for which.

2 This is the reason (THAT) I was telling you about --------这时候，about的後面没有it ,这说明，从句部分少了宾语，你就用that / which 或者省略that / which -----因为它们处在宾语的位置，可以省略。

现在你会比较了吗

Answer 2

why,根据语境，应该是why吧

Answer 3

填why ，这是一个定语从句，why是关系副词，意思是：这就是我告诉你这个的原因。

Answer 4

This is the reason __why__I was telling you about it.

这就是为什么我要告诉你（那件事）的原因。

Answer 5

that----限制性定语从句

Answer 6

That, 同位语从句,

I was telling you about it是对reason的补充和介绍