The correct answer is D. Bellary.
Bellary district is one of the richest districts in Karnataka in terms of mineral resources. It is the largest producer of iron ore in India and also has significant reserves of limestone, bauxite, and manganese.
Gulbarga district is also a major producer of iron ore, but it does not have as much limestone as Bellary. Bagalkot district is a major producer of cotton, but it does not have any significant mineral resources. Bijapur district is a major producer of sugar, but it does not have any significant mineral resources.
Limestone is a sedimentary rock composed mainly of calcium carbonate. It is used in a variety of industries, including construction, cement manufacturing, and steelmaking.