Which professional studies organisms and plant life to understand their composition and interactions with the environment?

The correct answer is rooted in the definition and scope of study associated with biologists. A biologist is a scientist who studies living organisms, which encompasses a wide range of subjects, including animals, plants, microorganisms, and their interactions within various ecosystems. Biologists delve into the composition, structure, function, growth, origin, evolution, and distribution of these organisms, seeking to comprehend how they interact with one another and their environment.

While other choices may concern specific aspects of this broad field, they do not encompass the full scope of what a biologist studies. For instance, a botanist focuses specifically on plant life, examining their biology and characteristics, but does not explore the full array of organisms studied by biologists. An ecologist would primarily concentrate on the relationships between organisms and their environments, which is a narrower field in comparison to the broader study conducted by biologists. An environmental scientist emphasizes issues related to the environment, potentially addressing human impacts and sustainability, but does not solely focus on the biological aspects of organisms.

Thus, the comprehensive nature of a biologist's work in understanding various organisms and their environmental interactions justifies their classification and makes them the correct answer in this context.