正确答案应该是D
译为：这些受灾的印第安人过去曾住在今天的美国.
这是what 引导的宾语从句,同时再从句中做主语
what is now part of the USA 做live in 的宾语
what=the place that
A 的话,该句子缺宾语,故不符合题意
B的话,应表达为
the Indians used to live in the place that is now part of the USA
what =the place that
C,这是宾从,改成先行词加which的定从可以,the place which
故选择D