The correct answer is: a) Treaty of Yandabo.
The Treaty of Yandabo was signed on 24 February 1826 between the British East India Company and the Burmese Empire. It marked the end of the First Anglo-Burmese War and resulted in the cession of Assam, Manipur, and Cachar to the British. The treaty also opened up the Burmese ports of Rangoon, Bassein, and Martaban to British trade.
The Treaty of Yandabo was a significant event in the history of British colonialism in India. It marked the beginning of British control over Assam, which would eventually extend to parts of Arunachal Pradesh. The treaty also had a significant impact on the Burmese Empire, which was forced to cede territory and open up its ports to British trade.
The other options are incorrect because they do not mark the beginning of British colonial influence in Assam. The Treaty of Simla was signed in 1914 between the British and the Tibetans. The Treaty of Gandamak was signed in 1879 between the British and the Afghans. The Treaty of Sagauli was signed in 1816 between the British and the Nepalese.