The major commercial center in South Sikkim is:

The correct answer is: d) Singtam.

Singtam is a town in South Sikkim, India. It is the headquarters of the West Sikkim district. Singtam is a major commercial center in South Sikkim. It is located on the National Highway 31A, which connects Gangtok to Siliguri. Singtam is also a major railway station on the North East Frontier Railway.

The town is well-connected by road and rail to other parts of Sikkim and India. It is also a popular tourist destination. The town is home to several temples, monasteries, and other historical sites. Singtam is also a good place to shop for souvenirs and local handicrafts.

The other options are incorrect because:

• Namchi is a town in South Sikkim, India. It is the headquarters of the South Sikkim district. Namchi is not a major commercial center.
• Ravangla is a town in South Sikkim, India. It is the headquarters of the East Sikkim district. Ravangla is not a major commercial center.
• Jorethang is a town in South Sikkim, India. It is located on the National Highway 31A, which connects Gangtok to Siliguri. Jorethang is not a major commercial center.