The correct answer is: A. Harnaut, Nalanda district
The Carriage Repair Workshop was set up in 2003 at Harnaut, Nalanda district. It is a major railway workshop in India. It is located on the Howrah-Gaya-Delhi main line, about 100 km from Patna. The workshop is responsible for the maintenance and repair of all types of railway carriages, including coaches, wagons, and locomotives. It has a workforce of over 2,000 employees.
Option B is incorrect because Jamalpur is a town in Munger district, Bihar. It is not located on the Howrah-Gaya-Delhi main line.
Option C is incorrect because Bihar Sharif is a town in Nalanda district, Bihar. It is not located on the Howrah-Gaya-Delhi main line.
Option D is incorrect because there is no railway workshop in India that is located at both Harnaut and Nalanda.