26th October, 1947 is an important date in the Indian history, because of

The 26th of October, 1947, is a pivotal date in Indian history as it marks the day Maharaja Hari Singh, the ruler of the princely state of Jammu and Kashmir, signed the Instrument of Accession [2]. Following the invasion by Pakistan-sponsored tribal militia on October 22, 1947, the Maharaja sought military assistance from India [t1][t9]. India maintained that troops could only be dispatched if the state legally acceded to the Indian Union [t9]. Consequently, the Maharaja executed the legal document on October 26, which was subsequently accepted by the Governor-General of India, Lord Mountbatten, on October 27, 1947 [c1][t6]. This act legally and constitutionally integrated Jammu and Kashmir into India [c1]. Other options are incorrect as the ceasefire with Pakistan did not occur until January 1, 1949 [c3], and the merger of Sind is not associated with this specific date.

Sources

1. [1] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 16: India–Political Aspects > The Kashmir Problem > p. 38
2. [2] Introduction to the Constitution of India, D. D. Basu (26th ed.). > Chapter 15: Jammu and Kashmir > Implications of the Accession. > p. 300