Variation in human skin color is an example of
a. incomplete dominance.
c. polygenic traits.
b. codominance.
d. multiple alleles.

The answer is c. polygenic traits.

Human skin color is a polygenic trait. That means that it is influenced by many genes. It is known few hundreds of genetic loci that are involved in the determination of skin color. 
Codominance could not be a correct answer because in that case both alleles should be expressed.
Incomplete dominance is not the cause of human skin color because phenotype would not be expressed if some environmental factors are present.
Although in multiple alleles there is more than just a single pair of alleles, it is not the cause of skin color variation. Polygenic traits influenced by more than one allele at different loci unlike the multiple alleles.