What lies between the sand dunes and the coast in the East Coastal Plains?

The correct answer is Lagoons.

In the East Coastal Plains of India, the region between the sand dunes and the coastline features distinct coastal landforms. Due to the depositional action of sea waves and rivers, shallow water bodies are partially or completely separated from the open sea by sandbars or barrier islands. These coastal features formed between the sand dunes and the sea coast are known as lagoons (also locally referred to as backwaters, such as Chilika Lake in Odisha or Pulicat Lake in Andhra Pradesh/Tamil Nadu).