The correct answer is: B) British treaties with rulers.
The British East India Company signed treaties with the rulers of Travancore and Cochin in the 18th century. These treaties gave the British control over the foreign affairs of these kingdoms, but they also allowed the rulers to maintain their internal autonomy. This meant that the British did not interfere in the day-to-day running of these kingdoms, and the rulers were able to keep their subjects relatively content.
The other options are not as likely explanations for the lack of widespread revolt in Travancore and Cochin. Option A, the presence of matrilineal systems, is not likely to have had a significant impact on the likelihood of revolt. Option C, religious tolerance, is also not likely to have been a major factor, as both Travancore and Cochin were Hindu kingdoms. Option D, geographical barriers, is also not a likely explanation, as both kingdoms were located on the coast and were therefore easily accessible to the British.
In conclusion, the British treaties with the rulers of Travancore and Cochin are the most likely explanation for the lack of widespread revolt in these kingdoms.