Final answer:
The variations in tortoise features on different Galápagos islands are examples of adaptations, which are traits allowing organisms to survive and reproduce in their specific environments.
Step-by-step explanation:
The variations in the features of the tortoise populations on Albemarle and Abingdon islands illustrate adaptations. Adaptations are traits that have developed over time, allowing organisms to survive and reproduce in their particular environments. The tortoises on different islands developed different physical characteristics as a response to their unique environmental conditions. For example, tortoises with access to ground plants have rounded shells and shorter necks, whereas those on islands with taller vegetation have longer necks and upward-bending shells. These are not vestigial structures, since those are remnants with little or no current function, nor analogous structures, which occur when different species independently evolve similar traits due to similar environments but without a common ancestor. Also, these variations are not due to molecular similarities, which refer to genetic commonalities. Hence, the correct option is A. adaptations.