－Why doesn't your friend buy you a drink?

D.because
你前面理解没有错,it is后面接表语或表语从句,这里是because引导表语从句不是状语从句.

because可以引导原因状语从句和表语从句,区别很简单,就看前面是不是it/that/this is.

附：that, why 与 because 引导表语从句时的区别
虽然三者均可引导表语从句,但 that 没有词义,而 why 和 because 有自己的意思；另外,虽然 why和 because 都可引导表语从句,但前者强调结果,后者强调原因.如：
The reason was that you don’t trust her. 原因是你不信任她.
The fact is that they are angry with each other. 事实是他们生彼此的气.
He was ill. That’s why he was sent to the hospital. 他病了,所以被送到医院来.
He was sent to the hospital. That’s because he was ill. 他被送到医院,是因为他生病了.

供参考,希望帮到你.