An accurate definition of tenebrism would be that it is marked by a very strong contrast between light and dark tones, imagine like when people tell ghost stories and everything's dark but they point a flashlight to their faces. The darkness is the dominating feature but the light is there as a sharp contrast. This is what tenebrism was and what Caravaggio did all the time.
I believe the answer is d) Unity. Since unity is what makes all elements in an artwork combine together well-harmoniously, I would think that'd be the most significant principle of a design according to photography.
