d
These are all “terms of art”, and hence not to be altered. That is, Saturn has a “ring system” not just “rings”, so you don’t edit it down to avoid repetition. Similarly, “particle density” is in contradistinction to mass density, for example, and is a property of the “ring particles” and not some other particles. I myself would have no objection to “particle density of the rings”, but I understand this kind of talk.