The correct answer is C. Delta.
A delta is a landform that forms at the mouth of a river where the river meets the sea. It is made up of sediment that is carried by the river and deposited as the river slows down and spreads out. Deltas are important because they provide habitat for many different plants and animals, and they also help to protect coastal areas from flooding.
Option A, ecologically sensitive areas, is incorrect because it is a general term that can refer to any area that is important for the environment. Option B, grasslands, is incorrect because grasslands are areas of land that are covered in grass. Option D, biologically sensitive areas, is incorrect because it is a general term that can refer to any area that is important for plants and animals.