All of the answer choices are grammatically correct, what is important is what the question is asking: the idea presented earlier in the sentence. Looking at the sentence, the author states “it is a philosophical debate rendered concrete”.
Answer choice A does not describe something philosophical becoming concrete, it actually does the exact opposite, so A is wrong.
In choice B, attitudes could be considered philosophical, but abstractions are most definitely not concrete, so B is wrong.
Choice C does the same thing, the philosophical idea does not become something concrete, so C is wrong.
D describes concepts (philosophical) becoming characters, which are indeed concrete things, so D is correct.