The correct option is It is widest in the Arctic Ocean (Siberian Shelf)..
Explanation
The continental shelf is the gently sloping seaward extension of the continental plate. It is the shallowest part of the ocean floor. The width of continental shelves varies significantly across the globe, largely determined by the tectonic setting of the coastline (active versus passive margins).
Option-wise Analysis:
- It is constant at 80 km worldwide. is incorrect: The width of the continental shelf is not constant. While the global average width is approximately 80 km, actual widths range from a few kilometers to over 1,000 km depending on the region.
- It is widest off the coast of Chile. is incorrect: The coast of Chile is an active continental margin where the Nazca Plate subducts beneath the South American Plate. Such regions are characterized by extremely narrow or non-existent continental shelves.
- It is widest in the Arctic Ocean (Siberian Shelf). is correct: The continental shelf is widest in the Arctic Ocean. The Siberian Shelf in the Arctic Ocean extends up to 1,500 km in width, making it the largest in the world.
- It is absent in the Arctic Ocean. is incorrect: The Arctic Ocean is not devoid of continental shelves; rather, it contains some of the most extensive shelf areas globally.
Key Takeaway: Continental shelves are typically wider along passive margins (e.g., the Arctic and Atlantic coasts) and narrower along active margins (e.g., the Pacific coast of South America). The Siberian Shelf holds the record for the maximum width.