The Bellary-Hospet region in Karnataka is primarily known for its reserves of:

The correct answer is A) Iron ore.

The Bellary-Hospet region in Karnataka is primarily known for its reserves of iron ore. The region is home to some of the largest iron ore deposits in India, and has been a major producer of iron ore for centuries. The iron ore from the Bellary-Hospet region is high quality and is used in a variety of industries, including steelmaking, construction, and manufacturing.

The other options are incorrect. Gold is found in the Kolar Gold Fields in Karnataka, copper is found in the Khetri Copper Belt in Rajasthan, and limestone is found in the Aravalli Range in Rajasthan.