Concept:Continuous variation results from the combined effect of many genes, each contributing a small additive effect.Explanation:Traits like height and skin colour show a wide, uninterrupted range of values.Such traits are controlled by multiple genes working together.This pattern is known as polygenic inheritance.Because each gene adds a small effect, many intermediate phenotypes appear.Therefore, continuous variation is not caused by a single gene or by simple dominance patterns.Answer:D. multiple genes