This question actually asks you to pick between working on studies that relate to humans, animals, or plants. Your answer indicates that you'd love to work with all of them, which is great, but it doesn't correctly answer the question you've been asked. Put some thought into what kind of studies you'd like to work on most - human, animal, or plant - and when you answer the question, focus on why you like that one the best.

You've confirmed that you prefer to work with human studies. You can give this answer more impact by talking briefly about any experience you have working with human studies and how that work contributed toward developing a new medical treatment or achieving a better quality of life for others. If you don't yet have experience working with human studies, you can share the known benefits of human studies and what skills this role would help you gain in order to achieve your ultimate goal of improving patient lives.

This is a great start because it's succinct and clear. You can give the interviewer more insight into what kind of work inspires and motivates you by talking about why human studies are the most interesting to you, why you think they're valuable, and how they help move the field of cardiovascular medicine or technology forward.