The doctor ________is leaving for America next month.
A the nurse is talking to him
B whom the nurse is talking
C the nurse is talking to
D who the nurse is talking

原因？谢谢。

The doctor ___C____is leaving for America next month.
A the nurse is talking to him
B whom the nurse is talking
C the nurse is talking to
D who the nurse is talking

这是一句定语从句,the doctor在句中充当talk to/with的宾语,所以关系代词who/whom可以省略,这不是排除选项的关键。
talk在表示为与人谈论时，是不及物的，这个to或者with不能省略，所以B、D均可排除，A的问题在于the nurse is talking to him从句是个完整的句子只能用关系副词，而who是一个关系代词，who在从句中指代the doctor，从句又出现him，有重复。