The Eastern Coastal Plains have well-developed deltas formed by rivers flowing eastward into the Bay of Bengal. Among them, the Godavari river is known for forming a significant and well-developed delta in this region.

The correct answer is option 'A', Godavari river. The Godavari river is known for forming a well-developed delta in the Eastern Coastal Plains. Let's delve into the details to understand why.

1. The Godavari River:
- The Godavari river is one of the major rivers in India, originating from the Brahmagiri Mountain in the Western Ghats of Maharashtra.
- It flows across several states, including Maharashtra, Telangana, Andhra Pradesh, and finally drains into the Bay of Bengal.
- The river has a total length of around 1,465 kilometers, making it the second longest river in India after the Ganges.

2. Formation of Delta:
- A delta is a landform that forms at the mouth of a river, where it meets a body of water, such as a sea or an ocean.
- Deltas are formed when sediment carried by the river settles and accumulates at its mouth, creating a triangular or fan-shaped landmass.
- The sediment consists of various materials like silt, sand, and clay, which are carried by the river from its source and then deposited when the river's velocity decreases upon entering the sea or ocean.

3. Godavari Delta:
- The Godavari river forms a well-developed delta as it reaches the Bay of Bengal in the Eastern Coastal Plains.
- The delta is spread across the districts of East Godavari and West Godavari in the state of Andhra Pradesh.
- It is one of the largest and most fertile deltas in India, covering an area of approximately 7,800 square kilometers.

4. Characteristics of Godavari Delta:
- The Godavari delta is known for its rich alluvial soil, which is highly suitable for agriculture.
- The region is characterized by rivers, distributaries, and numerous interconnected channels.
- It is home to a variety of flora and fauna, including mangroves, aquatic plants, and several bird species.
- The delta is also important for irrigation purposes, with several canals and reservoirs built to harness the river's water for agriculture and drinking water supply.

In conclusion, the Godavari river in the Eastern Coastal Plains is well-known for forming a well-developed delta. The river's sediment deposition over time has resulted in the formation of a fertile and agriculturally significant region, making it an essential geographical feature in the area.