The political party that formally accepted the Cabinet Mission Plan on 6th June, 1946, without the demand for a sovereign Pakistan was the Muslim League. The Cabinet Mission Plan was a proposal put forward by the British government to address the issue of India`s independence and the division of power between Hindus and Muslims. The plan suggested the formation of a united India with a decentralized government and the inclusion of representatives from both Hindu and Muslim communities. However, it did not meet the Muslim League`s demand for a separate nation for Muslims, which they had been advocating for in the form of Pakistan.

The Hindu Mahasabha, option 1, was a prominent right-wing Hindu nationalist organization, but they did not support the Cabinet Mission Plan. The Congress, option 2, was the largest political party in India and played a pivotal role in the freedom struggle, but they also did not completely accept the plan. The Unionist Party, option 4, was a political party mainly consisting of landowners and was more active in undivided Punjab.

In conclusion, the Muslim League was the political party that formally accepted the Cabinet Mission Plan on 6th June, 1946, while rejecting the demand for a sovereign Pakistan.