Correct option is D
The term "Seven Sisters" refers to the seven northeastern states of India, which include:
Meghalaya
Arunachal Pradesh
Assam
Manipur
Mizoram
Nagaland
Tripura
These states are called the Seven Sisters due to their geographic proximity and cultural similarities. Sikkim, although located in the northeastern region of India, is not part of the Seven Sisters. It became the 22nd state of India in 1975 but is geographically and politically distinct from the Seven Sisters