I hate to say that it is idiom because we can call most of things as idiom and its tough to spot uncommon idiom in a flash: I go by rules:
Requires cannot be followed by must as "requires" and "must" are reduntant..Rule out D & E
When you have command oriented word in a sentence and if it is followed by "that" then we use infinitive form of base verb without "to" i.e. to go, to make, etc..
When there is no such "that" following command words like requires, intend, instruct, imperative, direct, etc then the verb can be infinitve form i.e. it should appear with "to" e.g. to go, to play, etc.. This rules out A and B
So C