The correct answer is option 'A' - The President of India.

Explanation:
The delimitation order refers to the process of redrawing the boundaries of Lok Sabha and state legislative assembly constituencies in India. This process is carried out to ensure fair and equal representation of the people in the legislative bodies. The authority responsible for reviewing and approving the delimitation order is the President of India.

Below are the detailed explanations for each option:

a) The President of India:
The delimitation order, after being prepared by the Delimitation Commission, is submitted to the President of India for review and approval. The President has the authority to either approve or reject the order. Once approved, the delimitation order comes into effect.

b) The Prime Minister of India:
The Prime Minister of India does not have the authority to review or approve the delimitation order. The Prime Minister is the head of the government and is primarily responsible for the administration and implementation of government policies.

c) The Parliament of India:
The Parliament of India does not review or approve the delimitation order. However, the delimitation order is laid before both houses of Parliament for their information. This is a procedural requirement to ensure transparency and accountability in the delimitation process.

d) The Election Commission of India:
The Election Commission of India is responsible for the conduct of elections in the country. It plays a crucial role in the delimitation process by assisting the Delimitation Commission in preparing the delimitation order. However, the Election Commission does not have the authority to review or approve the order. Its role is limited to providing technical support and expertise.

In conclusion, the President of India is the authority that reviews and approves the delimitation order. The President's role ensures that the redrawing of constituency boundaries is carried out in a fair and unbiased manner, promoting equal representation of the people in legislative bodies.