Hello!
For background the plant shown is an orchid. Generally, purple is the dominant color and trait within this species and white the recessive allele.
If we use Mendell's Punnett Square we can actually see that together the pair are codominant with one another.
So the answer to your question would be, c. codominant with the allele for the white flower color.
If only one particular color was showing, we could say that one was dominant over the other. However, due to the fact that both traits show, that means they are codominant ! As recessive genes will not show under the presence of a dominant gene.
Hope this helps answer your question!