_____look is at the sight of the terrible scene!

A
frightening是形容物或事的,恐惧的...
frightened 是形容人的,令人恐惧的.
这里用frightening形容her frightened look.

首先，这里是个感叹句
What a pretty girl （she is）!
How pretty the girl is!
这是感叹句的两种表现形式。

接下来
frightening 令人恐怖的，用来修饰物
frightened 受惊的，受恐吓的，用来修饰人，表情
A frightened look 受惊的表情
因此，只有答案A是对的
How frightening her frightened look is at the sight of the terrible scene!
看到那恐怖的一幕，她受惊的面孔多吓人啊！

A.How frightening her frightened
这是一个感叹句，how ...is形式，后面的 at the sight of the terrible scene是定语。

look is at the sight of the terrible scene!
句子有误

A.

frightening形容情景
frightened形容人

D.What frightening her