He stood quietly at the door.His _______ (suddenly) appearance made his family very surprised.

划线处应填 sudden adj.突然的,意外的 ；
suddenly是副词（adv.）,用来修饰形容词（adj.）和动词（v.）,而本句中的appearance是名词（n.）,需要形容词来修饰,所以填suddenly的形容词形式sudden.
翻译：他安静地站在门口.他的突然出现使他的家人感到惊讶.