I hope that the little _____ I have been able to do does good to them all. A.which B.where C.when

这里没有答案， 答案应该是 that, 先行词为the little.
little 在这里是不定代词，和all 等词一样， 被定语从句修饰时关系代词不能是which, 而应该是that
I hope （主句主谓语）that the little宾语从句主语）【that】I have been able to do（修饰主语little的定语从句） does（宾语从句的谓语） good to them all（从句双宾语）.

全句意为：
我希望我的微薄之力能对他们所有的带来好处

【希望帮助到你，若有疑问，可以追问~~~
祝你学习进步，更上一层楼！(*^__^*)】

你好。最好的答案是 that. the little 已经是限定了，所以，如果有定语从句来修饰the little，应该用that而不是which来充当从句的宾语。 还有以下例子，一起记住吧：
the best THAT she can do
the worst day THAT we had there
the many people THAT she has met for today
the least possibility THAT there is of his survival in a bad car accident
the smallest house THAT we see here a family of ten lives in.

that
little在此是名词，that I have been able to do 是定语从句，修饰the little(指事物），而the little在从句中作宾语。