The correct answer is: b) British colonial rule.
The princely states in Assam enjoyed varying degrees of autonomy under British colonial rule. The British East India Company first established a presence in Assam in the early 18th century, and by the mid-19th century, the entire region was under British control. The British allowed the princely states to retain a degree of autonomy, but they were ultimately subject to British rule.
The Ahoms were a powerful kingdom that ruled Assam from the 13th to the 18th centuries. They were not a European power, and they did not rule the princely states in Assam.
Assam was an independent kingdom for much of its history, but it was conquered by the Mughals in the 17th century. The Mughals ruled Assam for over a century, but they were never able to fully subdue the local population. The Ahoms eventually regained control of Assam in the 18th century, but they were unable to prevent the British from taking control of the region in the 19th century.