In which of the following groups of districts is India’s largest mica belt…

Concept: Commercial mica is a sheet silicate that grows as large "books" inside pegmatite veins — the coarse, water-rich last fraction of a granitic magma. Such veins occur where ancient Precambrian granite and gneiss form the crust, so every major mica belt in India lies on an old shield or plateau block rather than in the alluvial plains or the young fold mountains.

Application: India has three principal mica belts — the Chotanagpur belt of Jharkhand and Bihar, the Aravalli belt of Rajasthan, and the Nellore belt of Andhra Pradesh. The largest of the three runs along the northern edge of the Chotanagpur plateau: it stretches about 150 km from Gaya through Koderma and Hazaribagh to Munger (older spelling Monghyr), is roughly 20-25 km wide, and yields the high-grade ruby muscovite on which India’s long dominance of world mica exports was built. Hazaribagh, Gaya and Monghyr are the districts that name this belt.

Cross-check — the other district groups:

• Balaghat and Chhindwara (Madhya Pradesh): a Satpura tract worked for manganese ore and for copper at Malanjkhand.

• Udaipur, Ajmer and Alwar (Rajasthan): the Aravalli mica belt, second in extent to the Chotanagpur belt, which also yields feldspar and the Zawar zinc-lead ores.

• Salem and Dharmapuri (Tamil Nadu): magnesite, bauxite and iron ore of the Salem hills, with granite quarried around Dharmapuri.

Hence India’s largest mica belt is found in the districts of Hazaribagh, Gaya and Monghyr.