The correct answer is: a) Treaty of Yandaboo (1826).
The Treaty of Yandaboo was signed on 24 February 1826 between the British East India Company and the Burmese Empire. The treaty ended the First Anglo-Burmese War and resulted in the cession of Assam, Manipur, and Cachar to the British.
The Battle of Plassey was fought on 23 June 1757 between the British East India Company and the Nawab of Bengal, Siraj ud-Daulah. The battle resulted in the British victory and the establishment of the British Raj in India.
The Battle of Buxar was fought on 22 October 1764 between the British East India Company and the Nawab of Awadh, Shuja-ud-Daula, and the Mughal emperor Shah Alam II. The battle resulted in the British victory and the consolidation of British power in India.
The Battle of Saraighat was fought on 4 December 1671 between the Ahom kingdom and the Mughal Empire. The battle resulted in the Ahom victory and the repulse of the Mughal invasion.
Assam came under the control of the British East India Company after the Treaty of Yandaboo (1826).