Concept:When all offspring of a cross show one parental trait, that trait is the dominant trait.Explanation:A red cock was crossed with a white hen, and all the F1 offspring were red.This shows that the red colour appeared in every offspring.The red trait therefore masked or hid the white trait.A trait that masks another trait is called the dominant trait.So, the gene for red colour was dominant over the gene for white colour.Answer:B. red colour was dominant