是What a clean room?还是 How clean room

What a clean room !是正确的.
这里主要涉及感叹句的知识:对名词感叹用what,对形容词或副词感叹用how.
其结构如下:
What (a/an)+adj.+n.+ 主语+谓动!
How +adj./adv.+ 主语+谓动!
如:What a beautiful girl she is !
What fine weather it was yesterday !
What happy children they are !
How beautiful the girl is !
How fine the weather was yesterday !
How happy they are !
How clean the room is !