The correct answer is C. buffer zone.
A biosphere reserve is a protected area of land or sea that is designated to protect biodiversity, support scientific research, and promote sustainable development. It is divided into three zones: the core zone, the buffer zone, and the transition zone.
The core zone is the innermost zone of a biosphere reserve. It is strictly protected and is off-limits to human activity. The purpose of the core zone is to protect the area’s biodiversity and ecosystems.
The buffer zone is the next outermost zone of a biosphere reserve. It is less strictly protected than the core zone, but it is still off-limits to most human activity. The purpose of the buffer zone is to protect the core zone from human activity and to provide a buffer between the core zone and the transition zone.
The transition zone is the outermost zone of a biosphere reserve. It is the only zone that is open to human activity. The purpose of the transition zone is to promote sustainable development and to provide a buffer between the biosphere reserve and the surrounding area.
In conclusion, the outermost zone of a biosphere reserve is the buffer zone.