The correct option is A pleiotropy
Gene which produces more than one phenotypic effect as in sickle cell anaemia gene is called pleiotropy.
In co-dominance, both the alleles produce it's effect simultaneously.
However, in incomplete dominance, none of the allele produces effect completely but produce intermediate phenotype.
In complete dominance, one allele called the dominant allele is expressed in both homozygous and heterozygous conditions while the other allele called the recessive allele is able to express only in homozygous condition.