The guests _______ their thanks before leaving.

A. showed B. expressed

本题选 expressed 是对的。根据《牛津英语搭配词典》，要表示“表达谢意；道谢”，thanks 主要搭配三个动词——express, give, say。如：

We would like to express our warmest thanks for all you’ve done. 我们想对你们所做的一切表示最热诚的感谢。

I said thanks and put the phone down. 我说了声“谢谢”就挂断了电话。

He gave thanks to God for the safe return of his son. 他为儿子的平安归来而感谢上帝。