The correct answer is: a) Bharuch.
Bharuch is a city in Gujarat, India. It is located on the banks of the Narmada River, about 100 kilometers from the state capital, Gandhinagar. Bharuch is a major industrial city, and is known for its production of marble, textiles, and chemicals.
The marble found in Bharuch is of high quality, and is used in a variety of applications, including building construction, flooring, and countertops. The marble industry is a major contributor to the economy of Bharuch, and employs thousands of people.
The other options are incorrect because they are not cities in Gujarat where significant reserves of marble are found. Rajkot is a city in Gujarat, but it is not known for its marble production. Ambaji is a town in Gujarat, and it is a popular pilgrimage site, but it is not known for its marble production. Vadodara is a city in Gujarat, and it is a major industrial city, but it is not known for its marble production.