What structure do sediments form at the end of a river?

Delta

Explanation:

At the end of the rivers, the water becomes much slower, and on top of it, it is entering another larger body of water. Because of this, the water loses its velocity, thus it is not able to carry the sediments anymore. The sediments are then deposited on a large scale at the end of the river, gradually forming a large area of soft, fine sediments. As the deposition of the sediments is becoming bigger and bigger, the waters of the river are blocked in certain parts, so the river spreads out into multiple smaller parts across the sediments, usually taking up a triangular shape. This natural formation form the sediments is called delta, and there are lot of examples around the world of it, some occupying very large areas, like the delta of the Nile River in Egypt.

a and b because with the other three, they simplify to something else.