I'm surprising that he () have been so foolish.
A must
B could
C should
D might

I'm surprising that he () have been so foolish.

C should


Should在高考中的考点
一、表示惊异、遗憾、不满等语气。
1.与疑问词how, why等连用，有"竟、会"之意。例如：
①Why should you stand there like a fool?你怎么像傻子一样站在那里？
(should+do表示现在的情况。)
②How should I know? Has it anything to do with me?我怎么知道？这和我有什么关系？
③I don't know why you should have told him that much.我不明白你为什么跟他说那么多。
(should+have done表示已发生的情况。)
2.在It's surprising/ strange/ a pity...以及I'm surprised/ sorry...等句型中，当"竟然会"讲。例如：
①It's surprising that you should be so foolish.真令人吃惊，你竟会如此愚蠢！(现在)
②It's surprising that you should have failed to pass such an exam.真令人吃惊，这样的考试你竟然不及格。(已发生)
3.单独用"should (not) +have+过去分词"形式，对已发生的事表示遗憾或责备，表达"本该或不该"之意。例如：
①He also learns that he should have cared more about his friends.他还明白了他本应该多关心朋友。(高一课本上册P4)
②He shouldn't have let out the secret.他本不该泄露秘密的。(实际情况是"他已泄密"。)