What is the answer to this problem?

Codominant and multiple alleles

Explanation:

Human blood types are an example of codominance, because both A and B alleles are expressed equally.

It cannot be complete dominance because neither the A or B allele are expressed over the other. I.e., neither are recessive.

It is not polygenic, as A B and O are multiple alleles of the same gene. Polygenic is when several genes control one trait. Therefore, it also represents a case of multiple alleles.

Sex linked traits are those present on the X chromosome, and we are told here that the gene is found on chromosome 9. Therefore it cannot be sex linked.

Incomplete dominance and codominance are similar but have different outcomes. instead of both alleles being expressed as in codominance, the resulting heterozygous phenotype is a 'blended' phenotype that is distinct from either homozygote. E.g. a red flower mated with a white flower producing pink flowers