Hm...I doubt NTR will happen as I dont really see a clear indication for that happening. Lets say she falls and no one saves her, she gets bitten (again) and turns into zombie. But what stops MC from having seggs with her to turn her back? Why exactly tells the douche will have to do it?
The other thing is like the other comment mentioned it mostly goes in two ways: Either everything is about MC getting cucked, or everything is save with some bit of baits. Sure, there are trash which try combine such stuff (Lust Geass), but that it got axed because of that is a good indication why not many try to repeat the same mistakes. On other hand there are also mangas which started "ok-ish", but turned into full blown japanse self-insert cuck fantasy (Guility Circle), but the difference between them is that in that mentioned manga the theme was not about MC or having him a Harem, but a weird fucking fuck fest with netorase fetish lurking in every chapter. This manga is the opposite.
So with that in mind I DONT see any NTR happning for the MC, let alone with the MAIN Love interest. However, experienced readers would say that since its japan authors you never truley know what might happen, so being catious is the smart approach. As for right now, I picked C as it would make more sense, as I dont see someone jumping in to save her.