Usha Mehta is an important figure in the history of the Indian Independence struggle and is most prominently known for option 1 - running the secret Congress Radio in the wake of the Quit India Movement. This shows her direct involvement with the Congress`s activities and her role in supporting the movement through communication channels.

Option 2 is incorrect as Usha Mehta did not participate in the Second Round Table Conference. This was a series of conferences held by the British to discuss constitutional reforms in India.

Option 3 is also incorrect as she did not lead any contingent of the Indian National Army, an armed force that was formed with the primary intention of overthrowing British rule in India during World War II.

Option 4 is incorrect as well. Usha Mehta did not assist in the formation of the Interim Government under Pandit Jawaharlal Nehru, which was established to lead India towards independence.

So, the correct answer is, indeed, option 1: she ran the secret Congress Radio during the Quit India movement.