No, I'm sorry, the answer D is not clearly right. Agreed, we all think that the answer the JAA expect is D, but the question has no correct answer. This is not an aerodynamic problem but a language communication problem.
Rene and others, Vmca is not a set of conditions, it is an IAS. It is defined, or demonstrated under certain conditions to show you that under these worst case emergency conditions you can maintain straight flight, but you can fly at, above or below the IAS known as Vmca if other conditions apply. Answer D only says "Straight flight cannot be maintained below VMCA when the critical engine has failed". So long as you remember that Vmca is just an IAS you can see that this is not true.
If this question came up in an exam I would expect the FTOs to appeal it. Questions ought to be clear, precise and unambiguous. This one is confused and badly written.
Dick