The correct answer is: a) Control over resources.
The feudatory states in Manipur were constantly at war with each other for control over resources such as land, water, and trade routes. This was due to the fact that Manipur is a mountainous region with limited resources, and each state was trying to secure its own survival.
Religious differences were not a major cause of conflict between the feudatory states in Manipur. While there were some religious tensions, these were generally not enough to cause war.
Succession disputes were also not a major cause of conflict between the feudatory states in Manipur. While there were some disputes over who should inherit the throne, these were generally resolved peacefully.
Personal rivalries were also not a major cause of conflict between the feudatory states in Manipur. While there were some personal grudges between leaders, these were generally not enough to cause war.
In conclusion, the most common cause of conflict between the feudatory states in Manipur was control over resources.