Which of the following describes a supporting ecosystem service?

The correct answer is that maintaining genetic diversity is indeed a supporting ecosystem service. Supporting services are fundamental to the functioning of ecosystems and provide the necessary conditions for other services to be delivered. Genetic diversity is crucial for ecosystem resilience, allowing species to adapt to changing environmental conditions and withstand diseases. This diversity underpins the stability of ecosystems, facilitates processes like pollination, and contributes to ecosystem productivity.

In contrast, the other options represent different types of ecosystem services. Providing recreational spaces falls under cultural services, as it pertains to the non-material benefits that people gain from nature, often enhancing well-being and quality of life. Water purification relates to provisioning and regulating services because it underscores the capability of ecosystems to manage natural resources and regulate particular processes. Carbon emissions reduction is also a regulating service, as it refers to the ecosystem's ability to mitigate climate change impacts by absorbing carbon dioxide from the atmosphere.

Thus, maintaining genetic diversity is essential because it supports the various interactions and processes that allow other ecosystem services to thrive, making it the correct identification of a supporting ecosystem service.