The best example of pleiotropy is

The correct option is B Phenylketonuria
Pleiotropy is a condition in which a single gene has multiple phenotypic expressions.

An example of pleiotropy is phenylketonuria, an inherited disorder that affects the level of phenylalanine, an amino acid that can be obtained from food, in the human body. It is a metabolic disorder, which occurs due to a defective gene for the enzyme phenylalanine hydroxylase.
Phenylketonuria is caused by a single gene defect but causes multiple effects such as mental retardation, hypopigmentation of hair and skin.

Skin colour is an example of polygenic inheritance, which is controlled by three genes. The darkest colour corresponds to the three dominant alleles (AABBCC) and the lightest skin colour corresponds to the presence of recessive alleles (aabbcc).

Color blindness is inherited as a recessive trait on the X chromosome. This is known in genetics as X-linked recessive inheritance. As a result, the condition tends to affect males more often than females

ABO blood group system in human beings is an example of dominance, co-dominance and multiple alleles. Blood group A and B show dominance, co-dominance is shown by blood group AB. In AB blood group, alleles for blood group A(IA) and blood group B(IB) are codominant, so characterised by both antigens.

So, the correct answer is "option B"