The sixth schedule of the constitution is not implemented in which of the following northeast states of India?

Manipur

The correct answer is Option (2) - Manipur

Sixth Schedule: This schedule provides special provisions for the administration of tribal areas in specific states.
States covered: Assam, Meghalaya, Tripura, and Mizoram.
While Manipur, and Arunachal Pradesh have significant tribal populations, they are not covered under the Sixth Schedule. Some of these states might have their own tribal area administration provisions, but they wouldn't be under the central Sixth Schedule.