What legal document allowed Princely States to join either India or Pakistan after independence?

The correct option is Instrument of Accession.

Explanation:

When the Indian Independence Act of 1947 ended British paramountcy over the Indian Princely States, these states were given the choice to accede to either the Dominion of India or the Dominion of Pakistan, or to remain independent.

The Instrument of Accession was the legal document executed by the rulers of the Princely States to accede to either of the new dominions. By signing this document, the ruler surrendered control over three key subjects—Defense, External Affairs, and Communications—to the Dominion government while retaining internal autonomy in other affairs.