The signing of this document meant that their state agreed to become a part of the Union of India. Which of the following is being referred to here?

Instrument of Accession

The rulers of most of the states signed a document called the 'Instrument of Accession' which meant that their state agreed to become a part of the Union of India.

The Instrument of Accession: It was a unilateral agreement signed by the rulers of princely states to join India or Pakistan after the end of British rule in 1947. By signing the Instrument of Accession, the princely state surrendered control over matters related to defence, foreign affairs, communication etc. to the Dominion of India or Pakistan, depending on their choice.