The correct answer is: a) Regulate entry into tribal areas.
The Inner Line Permit (ILP) system was introduced by the British in 1873 to regulate entry into tribal areas in Northeast India. The system was designed to protect the tribal people from outside influences and to prevent them from being exploited by outsiders. The ILP system is still in place today, and it is required for all non-tribal people to obtain a permit before entering the protected areas.
Option b) is incorrect because the ILP system was not introduced to promote trade with Tibet. The British did have trade relations with Tibet, but the ILP system was not designed to facilitate this trade.
Option c) is incorrect because the ILP system was not introduced to suppress tribal revolts. The British did have to suppress several tribal revolts in Northeast India, but the ILP system was not designed to do this.
Option d) is incorrect because the ILP system was not introduced to collect revenue. The British did collect revenue from the tribal areas, but the ILP system was not designed to do this.