The British East India Company entered Assam following the:

Battle of Plassey
Battle of Buxar
Treaty of Yandaboo
Battle of Saraighat

The correct answer is: c) Treaty of Yandaboo.

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, which had lasted from 1824 to 1826. The treaty forced Burma to cede control of Assam, Manipur, and Rakhine 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 was a decisive victory for the British, and it marked the beginning of British rule in India.

The Battle of Buxar was fought on 22 October 1764 between the British East India Company and the combined forces of the Nawab of Awadh, Shuja-ud-Daula, and the Mughal emperor Shah Alam II. The battle was a decisive victory for the British, and it gave them control of much of northern India.

The Battle of Saraighat was fought on 4 December 1671 between the Ahom kingdom and the Mughal Empire. The battle was a decisive victory for the Ahoms, and it prevented the Mughals from conquering Assam.