From the perspective of someone investigating abuse, I think it's important that 'not an exit relay' means 'not capable of exiting on any port at all'. Ergo I think your option c) is the way to go.
I also think this (c) is the best option. I agree that it's important to be able to determine, from an investigatory perspective, whether or not a relay was capable of exiting on any port.
Regards,
Joshua Lee Tucker @tuckerwales