Which of these is a distinguishing abiotic feature of the great barrier reef

The Great Barrier Reef in Australia is known as the largest coral reef in the world and the largest living thing on Earth where it houses countless species of sea animals. 

Its distinguishing or distinct abiotic feature is its shallow waters.

Sunlight is one of the most important factors in an ecosystem necessary for the organism's survival. The reef mostly has shallow waters so it is easier for the light to penetrate the water for the organisms to use. Even the reefs are visible from the outer space due to its shallow waters. 

A. radioactive dating