With reference to the diagram given in the question, the feature labeled as A is:
An open ocean is the largest marine environment present on the planet earth. It begins from the edge of continental shelf and moves away from the shore. Therefore, option (a) is correct.
Coastal Ocean is a part of marine ecosystem which falls in between the shore and the edge of continental shelf. Here light penetration is highest and results in the high photosynthetic activity. Therefore, option (b) is incorrect.
A trench is the deep channel in the ocean floor that is generally found in the open ocean. Therefore, option (c) is incorrect.
Estuary is formed as the result of intermixing fresh water from the river and the salty water from the sea. This water is generally referred to as the brackish waters. Therefore, option (d) is incorrect.
Hence, the labeled feature A in the given diagram refers to (a) open ocean.