What is a delta?

A delta is specifically defined as a landform that forms at the mouth of a river, where it meets a body of water such as an ocean or a lake. This geographical feature is typically triangular or fan-shaped due to the sediment deposited by the river as it slows down and spreads out upon entering the larger body of water. Deltas are vital ecosystems that often support diverse wildlife and can be areas of rich soil ideal for agriculture, influenced by the nutrient-rich sediments carried by the river.

The other options do not accurately define what a delta is. A volcanic landform refers to structures created by volcanic activity, and while they can shape the landscape dramatically, they do not share characteristics with a delta. Ancient settlements generally refer to communities established long ago by human populations, which could be located in various terrains but are not specific to the characteristics of a delta. A fortified city implies a populated area that is defended by walls or military structures, which again is unrelated to the natural formation of a delta. Thus, the definition of a delta as a triangle-shaped area of land is not only correct but also highlights its significance in geography and ecology.