The video cameras are so nice.I really don't know-------.

【答案】 D.which to use
I really don't know which to use我不知道选哪一个
which充当know的宾语
which既作代词,又有它本身“哪一个”的含义,所以不用what how
不定式to choose作后置定语修饰宾语which
which也能引导宾语从句,但这句不是【这句俗称简单句】,若改成宾语从句,如下：
I really don't know which video I should choose.