Final answer:
The single-gene trait example is fur color in cats, which is determined by specific alleles at one locus. The fit of African and South American continental edges is due to continental drift, a result of plate tectonics.
Step-by-step explanation:
An example of a single-gene trait is D. fur color in cats. Single-gene traits are determined by the alleles at a single locus in the genome and have a clear and distinct phenotype. Other options such as bands on a snail's shell, beak size in Galápagos finches, and snail length typically result from complex interactions among multiple genes and environmental factors.
The reason why the edges of Africa and South America fit together like puzzle pieces can be explained by C. continental drift. This is the movement of the Earth's continents relative to each other due to plate tectonics.