When you're asked to supply a Modifier, as you are here (clues: it follows a comma and is nonessential to the meaning of the sentence - you could just as easily stop at "Pilsudski" and the sentence s till makes sense) it's important that the modifier is clear in what it described.
"Demanding" in A and B could describe "the newly formed state" - remember, "-ing" verbs used as modifiers can describe the subject of the sentence. That's fairly illogical (while being routed does it have any potential to demand anything?), and it also means that "it" as a pronoun is illogical ("it" really needs to refer to "Russian state", and in that case it wouldn't make sense to demand anything from itself).
C uses the pronoun "their", which has no referent - "their" is plural and the only nouns that it could refer to are "state" and "General".
Similarly, "E" uses "them", which has no referent. As Amma points out, D is correct: "who" as a modifier matches with "Jozef", and "it" to the state. The modifier and the pronoun both have clear, logical referents, and so D is correct.