Final answer:
Continuous variation in a trait is most likely when multiple genes with additive effects control it, a form of polygenic inheritance. Traits such as height and skin color exemplify this variation.
Step-by-step explanation:
A trait is most likely to exhibit continuous variation if it is controlled by multiple genes with additive effects. This is referred to as polygenic inheritance, where several genes contribute to a particular trait. Examples of polygenic traits include height, skin color, and weight, which show a wide range of variation and can be measured. These traits are influenced by the combined effect of multiple genes along with environmental factors.
Option (c) Multiple genes with additive effects is the correct. These polygenic traits contrast sharply with traits controlled by a single gene, which typically result in a trait with less variation and can display Mendelian inheritance patterns, like the cleft chin example.