Why the answer "how they can be so stupid" isn't allowed?
It's because your word order in english is wrong, because in Eng, the correct word order is "How can they be so stupid."
Your translations are not correct, you need more possible answers, hloupy is silly, even stupid, not generally crazy
"that stupid"? Why "that"? Why don't "so"?
It means the same.
"How can they be such stupid" is not possible?
"... such stupid people" or some other noun, otherwise it does not make much sense to me.
basically "such stupid .." - "takoví hloupí ..."
Why is it hloupý and not hloupé, since mohou implies that the subject of the verb, what is being modified by the adjective, is plural?
hloupí is masculine animate plural nominative
hloupé is masculine inanimate or feminine plural nominative
How would you say, "How could they be so stupid"?