The sentence is almost correct, but there is a small issue with part C. The phrase "the more dangerous it becomes to walk on" should ideally be phrased as "the more dangerous it becomes to walk on it" for clarity. However, in the context of common English usage, one might argue that the sentence as it stands is acceptable.

That said, if we must choose from the options provided, part D ("on") might be more appropriate to address the lack of specificity. The correct answer would remain debatable, but let's go with:

**D. on**

as the choice to improve clarity in the context of the sentence.