The Maharaja of Kashmir signed the Instrument of Accession to India on

On 26 October 1947 Maharaja Hari Singh of Kashmir signed the Instrument of Accession to India following tribal incursions from Pakistan. This decision allowed India to send troops into the state to defend against the attack. The document became the legal basis claimed by India for Kashmir accession though Pakistan disputed its validity. This accession is central to the long standing conflict over the region. The date recognized historically is 26 October 1947 which aligns with option C.