The correct answer is: C. both (a) and (b)
The continental margin is the part of a continent that extends underwater to the edge of the continental shelf. It is divided into two parts: the continental shelf and the continental slope. The continental shelf is the shallowest part of the continental margin, and it is typically covered by water that is less than 200 meters deep. The continental slope is the steeper part of the continental margin, and it descends from the continental shelf to the abyssal plain.
The continental shelf is structurally a part of the continent, as it is made up of the same continental crust as the continent itself. The continental slope, on the other hand, is not structurally a part of the continent, as it is made up of oceanic crust.
The continental shelf is important for a number of reasons. It is a major fishing ground, and it is also a popular destination for recreation, such as swimming, boating, and fishing. The continental slope is important for a number of reasons as well. It is a major source of oil and gas, and it is also a major area for submarine landslides.