The correct answer is option 'B': An Isthmus.

An isthmus is a narrow strip of land that connects two larger landmasses and is surrounded by water on two sides. In the case of North and South America, the isthmus that connects them is called the Isthmus of Panama.

Explanation:
The Isthmus of Panama is a narrow strip of land located in Central America. It connects the continents of North America and South America, acting as a land bridge between them.

Geographical Location:
The Isthmus of Panama is located in the country of Panama, which separates the Caribbean Sea to the north from the Pacific Ocean to the south. It stretches for approximately 50 miles (80 kilometers) at its narrowest point.

Impact:
The formation of the Isthmus of Panama had a significant impact on the Earth's biodiversity. It allowed for the exchange of flora and fauna between North and South America, leading to the Great American Interchange. This event resulted in the migration of various species across the isthmus, shaping the diversity of both continents.

Alternative Options:
Let's briefly discuss the other options mentioned:
a) Mountain: While there are mountain ranges in North and South America, they do not directly link the two continents. Mountains can act as natural barriers between regions.
c) Strait: A strait is a narrow body of water that connects two larger bodies of water. It does not connect two landmasses directly.
d) Canal: A canal is a man-made waterway that is constructed to connect two bodies of water. The Panama Canal, located in the Isthmus of Panama, is an example of a canal, but it was built after the isthmus already existed.

Conclusion:
The correct answer is option 'B': An Isthmus. The Isthmus of Panama connects the continents of North America and South America, allowing for the exchange of flora, fauna, and human activities between the two continents.