(A) Pleiotropic genes have multiple phenotypic effect.

(B) Muliple alleles exhibit same phenotypic expression.

(C) Polygenes exhibit continuous variation.

The correct option is D Statement (A), (C) are correct and (B) is incorrect.
Multiple alleles is a type of non-Mendelian inheritance pattern that involves more than just the typical two alleles that usually code for a certain characteristic in a species. With multiple alleles, that means there is more than two phenotypes available depending on the dominant or recessive alleles that are available in the trait and the dominance pattern the individual alleles follow when combined together.
Gregor Mendel only studied traits in his pea plants that showed simple or complete dominance and had only two alleles that could contribute to any one trait the plant showed. It wasn't until later that it was discovered that some traits can have more than two alleles that code for their phenotypes. This allowed many more phenotypes to be visible for any given trait while still following Mendel's Laws of Inheritance.
Most of the time, when multiple alleles come into play for a trait, there is a mix of types of dominance patterns that occur. Sometimes, one of the alleles is completely recessive to the others and will be masked by any of those that are dominant to it. Other alleles may be co-dominant together and show their traits equally in the phenotype of the individual. There are also some cases where some alleles exhibit incomplete dominance when put together in the genotype. An individual with this type of inheritance connected to its multiple alleles will show a blended phenotype that mixes both of the alleles' traits together.
Pleiotropy occurs when one gene influences multiple, seemingly unrelated phenotypic traits, an example being phenylketonuria, which is a human disease that affects multiple systems but is caused by one gene defect.