Thanks for the question! So the issue with (C) is that although the argument is in a way causal, it doesn’t do exactly what (C) says. Is the argument used to provide a causal explanation for an observed phenomenon? No, not really. That would be like if we had some kind of observed phenomenon like “the sky is blue” or something. And then the argument was used to explain the cause of the sky being blue (so one such argument would have to do with the diffraction of light). That’s an example of what (C) would look like, and it’s not going on here, which is why (C) is out.
Hope this helps! Feel free to ask any other questions that you might have.