Question

He stood quietly at the door. His _______ (suddenly) appearance made his fam

Answer 1

你好！
appearance 是名词，用形容下修饰，所以是suddenly的形容词形式为sudden。
He stood quietly at the door. His sudden appearance made his family very surprised.
他静静地站在门前。他的突然出现使得家人都十分惊讶。
仅代表个人观点，不喜勿喷，谢谢。

Answer 2

填
suddenly的
形容词sudden
修饰appearance
He
stood
quietly
at
the
door.
His
sudden
appearance
made
his
family
very
surprised.
翻译：
他静静站在门前。他的突然出现令他的家人很吃惊。