Yeah, (C) is a pretty weird one to diagram. I think I would just list out two of the premises, honestly, so it'd look like:
P1) Permissible to sit on park benches P2) Sit on park benches - > Walk to park benches P3) Can walk through grass Conclusion: Permissible to walk on grass
Not a super satisfying diagram, but probably close to what I'd actually do if I had to diagram it. Hope this helps!