The new American president's winning speech is a _____ that

The new American president's winning speech is a _____ that major changes will be on the way
The new American president's winning speech is a that major changes will be on the way.
A.warning B.goal C.signal D.suggestion
请问这里D怎么不可以呢?不也是暗示,表明的意思吗?
牛津高阶字典上有一个例句 在1529页 There is no suggestion that she should resign.没有任何迹象显示她要辞职。表明是可数名词。我很奇怪suggest表示暗示，表明不是用陈述语气吗？这里怎么后面还是用should do呢？