When he was seeing the boring film ,he was very disappointin

我可以很负责任的告诉你,答案就是disappointed.
老师讲的不一定都对,卷子答案有时候也会出错.
简单的说,
disappointed 是形容因为某人因为某事而失意,沮丧,主语基本上是人（或有时候拟人手法也会有其它事物) He was disappointed by the disappointing result.
disappointing 所指主语,本身具体令人失意,沮丧的特征,它不能作为进行时用,因为不是连续动词.
但可做形容词,比如disappointing result,指这个result本身就具体这个让人失望的特征.
举例说the result was disappointing.
the result 就类似" the boring film",前面半句是一个因,后面是一个后,主语"He"是被这件事搞得失意.
excited,exciting也是这个道理,举个例子:
When he heard the exciting news,he was excited.