The question asks which of the given rivers forms a delta along the eastern coastal plains of India. Let's analyze the options to understand which river fulfills this criterion.

1. Narmada: The Narmada river flows westward and ultimately drains into the Arabian Sea, forming an estuary rather than a delta. Hence, it does not form a delta along the eastern coastal plains.
2. Tapi: Similar to the Narmada, the Tapi river also flows westward and drains into the Arabian Sea, forming an estuary. It does not contribute to the delta formation on the eastern coast.
3. Ganga: The Ganga river forms a large delta, commonly known as the Sundarbans, in the eastern part of the Indian subcontinent. However, it is primarily in the region where it reaches the Bay of Bengal, and not specifically within the eastern coastal plains mentioned in the context.
4. Godavari: The Godavari river is the correct answer. It flows across the Deccan Plateau towards the Bay of Bengal and forms a large delta along the eastern coastal plains of India. The delta formed by the Godavari is one of the largest in India, and it is located on the eastern coast, consistent with the requirements of the question.

Conclusion: Among the given options, the Godavari river forms a delta along the eastern coastal plains. This is due to its eastward flow towards the Bay of Bengal, where it diverges into different distributaries to form the delta region.

Correct Answer: Godavari