The correct answer is: a) British Residents.
British Residents were appointed to the courts of Indian princely states to advise the rulers on matters of administration and to ensure that they did not act in a way that was contrary to British interests. They had a great deal of power and influence, and the rulers were often obliged to follow their advice.
Large military garrisons were also stationed in the princely states to provide security and to deter any uprisings. However, they were not as important as the British Residents in maintaining British control.
Democratic assemblies were not established in the princely states until after Indian independence. They had no role in British rule.
Missionary schools were established in some of the princely states, but they did not have a significant impact on British control.
In conclusion, the British maintained control over feudatory states through the presence of British Residents.