Which one of the following foreign travellers elaborately discussed about diamonds and diamond mines of India?

Tavernier is the right answer. He was a French jeweller who made six voyages to India in the mid-seventeenth century, and his Travels in India gives a detailed account of the diamond mines of Golconda and Kollur, the gem trade, and the cutting and polishing methods. He also describes stones he sold to Mughal and Persian courts, which is why his account is so often cited.

Why the others fall short:

• Bernier was a French physician who spent about twelve years at the Mughal court under Aurangzeb. His Travels in the Mughal Empire is a major source on agrarian economy and the Mughal nobility, but not on mines.
• Thevenot compiled travel accounts covering India, Persia and the Levant. He touches on many subjects but does not give the sustained gem-trade treatment that Tavernier does.
• Abbe Carre travelled in the early eighteenth century, and his letters record Mughal and Maratha politics of his time, not the mining economy.

Core concept: foreign travellers as sources for pre-colonial Indian economy, especially the trade in precious goods. For Mains, this links to Golconda's role in the global diamond trade and the wealth of the Deccan sultanates.

Trap: candidates pick Bernier simply because he is the most famous name in the list. Remember that the question asks about one specific subject, and Tavernier is the traveller whose work is built around gems.