The Haripura Congress held in 1938 is considered a significant milestone in the Indian freedom struggle due to the introduction of the idea of a planning commission. This option emphasizes the importance of the event in bringing about a new approach towards economic planning and development.

Option 1, stating that the Haripura Congress declared war on the British Empire, is incorrect. The Congress did not declare war on the British Empire during this event. It focused more on the socio-economic and political aspects of India`s struggle for independence.

Option 2, claiming that the Haripura Congress anointed Jawaharlal Nehru as the future Prime Minister of India, is also incorrect. While Nehru played a prominent role in the Congress, the event did not specifically appoint him as the future Prime Minister.

Option 4, suggesting that the Congress accepted the Government of India Act, 1935, is incorrect as well. The Congress did not fully endorse the Act but instead used it as a basis for demanding further reforms and pushing for complete independence.

Overall, the introduction of the idea of a planning commission stands as the most accurate and significant reason why the Haripura Congress is considered a milestone in the Indian freedom struggle.