. I don✀t know ______. A how to do B what to do C where to do D when to do

疑问词+ to do其实就是宾语从句的省略：
I don't know [when to do it].=I don't know [when I should to do it].

I don't know [where to do it].=I don't know [where I should do it].

I don't know [how to do it].=I don't know [how I should do it].

I don't know [What to do].=I don't know [What I should do].

可以看出do的主语都是I，同时do需要宾语[when，where，how都是疑问副词不能做do的宾语]

所以需要it出现做do的宾语；但是what是疑问代词可做do的宾语，不再需要it
----------英语门工厂

除了C, 其它答案都可以。
I don't know 【when to do】it.我不知道什么时候做那件事
I don't know 【where to do】it.我不知道在哪做那件事
I don't know 【how to do】it.我不知道怎么做那件事

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