The correct answer is option 4: Hindu Mahasabha. The Hindu Mahasabha was an organization that was started at the Haridwar Kumbh Mela in 1915. The Kumbh Mela is a Hindu pilgrimage festival held every 12 years in Haridwar, a city in Uttarakhand, India. The festival is considered to be one of the largest gatherings of people in the world.

The Hindu Mahasabha was founded as a political organization with the objective of promoting and protecting the interests of Hindus. It aimed to unify Hindus and safeguard their rights in a predominantly Hindu society. The organization played a significant role in Indian politics during the pre-independence era and after.

Option 1, Sanatan Dharma Sabha, is not the correct answer. Sanatan Dharma Sabha is also a Hindu organization, but it was not started at the Haridwar Kumbh Mela in 1915.

Option 2, Dev Samaj, is also not the correct answer. Dev Samaj is a religious reform movement founded by Shiv Narayan Agnihotri in the late 19th century, but it was not started at the Haridwar Kumbh Mela.

Option 3, Brahmin Sabha,