could you tell me ( )? what are they doing that for, why they are doing for ,why are they doing that, what they are doing that for 为什么选d

宾语从句必须用陈述句语序b,d符合。b句是个错句，why是副词不能做for 的宾语，所以选d.

Could you tell me+疑问词+宾语从句,宾语从句就是主语+谓语+宾语,也就是What is your name?变为Can you tell me what your name is?所以Could you tell me why are they doing that?是错的，应改为Could you tell me why they are doing that?

因为是要带从句,所以what are they doing that fo 和 why are they doing that是错的,要用陈述语序.why提问原因,不能与for搭配,只能选what they are doing that for

首先 could you tell me .....这个结构后面的从句要用 陈述语气 因此 排除了 A C
其次 why是表示原因
doing that for 介词后面需要跟宾语 构成 介词宾语短语，而这个宾语又要提到前面做连接词 D项中的what可以表示这个功能