Final answer:
Ichthyosaurs and mosasaurs adapted to become more aquatic by evolving the ability to give live birth in water, unlike plesiosaurs, which had to lay eggs on land. This adaptation allowed them to thrive in marine environments. Paddles helped in swimming but weren't the primary adaptation for increasing aquatic lifestyle.
Explanation:
Adaptations of Aquatic Reptiles
The question regarding which adaptation allowed ichthyosaurs and mosasaurs to become more fully aquatic than plesiosaurs points specifically to how these creatures evolved to thrive in marine environments.
Among the given options, the correct answer is C: the ability to give live birth rather than laying eggs on land. This adaptation allowed ichthyosaurs and mosasaurs to reproduce in the water, facilitating a fully aquatic lifestyle.
Unlike plesiosaurs, which still had to lay their eggs on land, giving live birth liberated these marine reptiles from the constraints of terrestrial reproduction. This means they could venture into deeper waters without the need to return to land for breeding.
Other Relevant Adaptations
While other adaptations such as the development of paddles for swimming are significant for locomotion in water, they do not necessarily correlate with becoming more fully aquatic as a species in the same way that birthing methods do.
Learn more about aquatic adaptations here:
https://brainly.com/question/40377558