The condition when the two sexes of the same species have different characteristics apart from difference in their Sexual dimorphism is known as Sexual diamorphism. It occurs in many plants and animals. The difference can include size, weight, colour, secondary sexual characteristics. The differences can be exaggerated or subtle. Monomorphism is the opposite of diamorphism.
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.