All the students were against the war.
I was curious about the en and discovered that the entire phrase en contra de here means against as an opinion, vs. the single word contra meaning against as a location, e.g. against a wall.
So it seems to me the entire phrase should be the “blank”, and include both en contra de and contra alone as possible answers.
If that were possible from a technical standpoint (I don’t know the way clozemaster works allows for that), and you removed “en” and “de” to allow “en contra de” as the correct answer, you shouldn’t count simply “contra” as correct (at least in this sentence). Because, as you rightly pointed out, “contra” alone implies a certain contact with the object you are against, and, since war is more of a concept (or an action, but definitely not an object you can be touching) you can’t really be in physical contact with it