Well, there's a chilling sentence.
how do you know that there are many criminals in this sentence.
"Political criminals" here is a general noun. So it should be plural.
When I saw "the criminal" as a previous exercise, it didn't have a classifier either, so how can we tell when a noun is supposed to be general? Context?
I think it is not neccessarily a general statement. If you have one specific political criminal in mind, you could use the same wording. If this is true, "The political criminal etc." is a correct translation, but was marked wrong.