The phenomenon by which male and female sexes could be differentiated morphologically is called

A. Variation
B. Polymorphism
C. Sexual dimorphism
D. none of these

C. Sexual dimorphism

To find the answer to the question, we can break down the options provided and analyze each one:

A. Variation: Variation refers to differences or diversity within a population. While variation may include differences between male and female individuals, it does not specifically refer to the differentiation of sexes based on morphology.

B. Polymorphism: Polymorphism refers to the occurrence of different forms or morphs within a population. Again, while polymorphism can occur within a population, it does not specifically address the differentiation between male and female sexes.

C. Sexual dimorphism: Sexual dimorphism is the correct answer. It is the phenomenon by which male and female sexes can be distinguished based on morphological differences. These differences could include variations in size, shape, coloration, or other physical characteristics.

D. None of these: As explained above, option C, sexual dimorphism, is the correct answer. Therefore, option D is not the correct choice.

So, the correct answer to the question is C. Sexual dimorphism.