The reason()her not coming was that she missed the bus.

C

reason for: …的原因(理由等)

例句与用法:
1. The reason for the change is lack of money, pure and simple.
进行更改的原因就是缺少经费.
2. That is one reason for my having entered you at Riverview.
这是我让你上河景中学的原因之一。
3. The reason for his success is not far to seek he worked hard.
他成功的原因不难找，那就是他勤奋工作。
4. I could give you the hundredth reason for not going.
我能给你讲出第一百个不去的理由。
5. What was the real reason for your absence?
你缺席的真正原因是什么？
6. The primary reason for advertising is to sell more goods.

选C吧