Tortious Interference with Contract: Where's the Line?

I'm discussing this with my friend, but we can't agree on MCQ 3 from Mulla's chapter on Liability for Interference with Contractual Relations. Here it is:

"If an unlawful act of A has caused B to break a contract with C, A will be liable for trespass if (a) he knew or had reason to believe that B would break the contract, (b) the unlawful act of A was not merely an innocent mistake, or (c) the unlawful act of A was not merely trespass on private property."

My friend says the correct answer is (b), but I think it's (c). My friend cites Ruiya v. East India Railway Co. where the court held that trespass on private property was not intentional. I'm like, what about the intention behind the act? Doesn't that matter?

You decide!