​Choose the word which is different from the rest.

Lets check all options;

Option (a): Chennai (False)

Chennai is a major city and the capital of Tamil Nadu in India. It is a metropolitan city with a large population.

Option (b): Singhbhum (True)
Singhbhum is a district located in the state of Jharkhand, India. It is more of a geographical region rather than a city, known for its rural and urban areas.
Option (c): Hyderabad (False)
Hyderabad is a major city and the capital of Telangana and Andhra Pradesh. It is a large metropolitan city.
Option (d): Gangtok (False)
Gangtok is the capital of Sikkim, which is a state in northeastern India. It is a smaller city in comparison to the major metropolitan cities like Chennai and Hyderabad.​​

So option (b) is different in all options.

Thus, the correct answer is (b).