All the options you list seem reasonable to me. The effective differences are subtle, and it's hard to say which is better. C might seem the most permissive, an

All the options you list seem reasonable to me. The effective differences are subtle, and it's hard to say which is better. C might seem the most permissive, and is the one I'd personally lean towards, but it also forces students to guess what is allowed, and face sanctions if they get it wrong, so they might end up self-censoring more harshly than if the rules were explicit. So while I prefer the spirit of C, in practice I'd choose B - if there's censorship, it should be out in the open. I'd also prefer if none of the choices dominated all universities, to avoid a monoculture.