One pair of alleles controls eye color in fruit flies. More than ten different eye colors are possible, ranging from bright red to apricot to tan to white. What kind of inheritance is this? How do you know?
It is an example of multiple alleles.
Multiple alleles occurs when three or more alternative forms of a gene (alleles) can occupy the same locus and only two of the alleles can be present in the organism. In this example there is only one pair producing ten colors.
It's not Incomplete dominance or co-dominance because each one of them can produce only three different phenotypes.
It is't also a polygenic inheritance as it can produce many phenotypes, but by more than one pair of alleles.
In this case, there is only one pair of alleles, so it has to be multiple alleles, with a different combination of alleles producing each different color.