请教一道关于英语成分的问题（定语从句中）?
The doctor__________is going to Europe next year.
A.which the manager is talking to
B.who the manager is talking
C.the manager is talking to him
D.the manager is talking to
材料的答案是D,但是我对这句句子的句子成分不是很清楚,不知道D选项中省略了什么关系词,我认为这个短语是做“the doctor”的定语,可是……请教知情者,谢谢