this the reason___you are in favour of the proposal?AwhichBwhatCwhyDfor that

this the reason___you are in favour of the proposal?AwhichBwhatCwhyDfor that
this the reason___you are in favour of the proposal?
Awhich
Bwhat
Cwhy
Dfor that

this the reason___you are in favour of the proposal?AwhichBwhatCwhyDfor that
选C 这是定语从句、要添的是关系代词、而往往和reason 连用的就是why 只要确定是定语从句、有reason 就选why

C the reason why ________后面加的是reason所导致的结果。
the reason that 只能作定语从句吧好像。

C