The region of biosphere reserve that is protected legally, where no human activity takes place is known as

The correct option is core zone.

To understand why this is the correct answer, let us look at the structure and zoning of a Biosphere Reserve. A Biosphere Reserve is divided into three distinct zones, each having a different level of protection and permitted human activity:
1. Core Zone: This is the innermost, legally protected area of a biosphere reserve. It consists of an undisturbed, secure ecosystem. No human activity of any kind (such as tourism, settlement, or resource extraction) is allowed here, ensuring the complete protection of wildlife and natural habitats.
2. Buffer Zone: Surrounding the core zone is the buffer zone. Here, only limited, non-destructive human activities are permitted. These include scientific research, environmental education, and low-impact tourism.
3. Transition Zone: This is the outermost part of the biosphere reserve. It is an area of cooperation where conservation efforts and sustainable human activities (like agriculture, forestry, and settlements) coexist harmony with the help of local communities.

Therefore, the strictly protected legal region where absolutely no human activity is allowed is the core zone.