Muzaffarpur is the leading litchi district — option 3.

The Shahi litchi. The variety grown around Muzaffarpur is called Shahi litchi, prized for its aroma, its small seed and its high pulp content. It received a Geographical Indication tag in 2018 — the fourth product from Bihar to be given one, after Jardalu mango, Katarni rice and Magahi paan. Bihar accounts for a large share of India’s litchi output, and India is among the world’s largest producers after China.

Why the crop concentrates there. Litchi needs deep, well-drained calcareous alluvial soil, high humidity, and a dry period before flowering — conditions the north Bihar plain along the Gandak provides. The fruit is highly perishable, ripening in a short season and deteriorating within days, which is why cold storage, processing and air freight matter so much to the trade and why so much of the crop is sold locally.

A public-health footnote. Muzaffarpur was the centre of outbreaks of acute encephalitis syndrome among children in the litchi season. Research traced the cause to hypoglycin A and MCPG, toxins in unripe litchi, which block the body’s ability to make glucose overnight; children who ate unripe fruit and went to bed without a meal were the ones affected. Ensuring an evening meal sharply reduced the deaths — a striking instance of epidemiology solving a long-standing mystery.

The other options are not even districts: Sonpur is a town in Saran, famous for the cattle fair, and Chhapra is the headquarters of Saran district. Only Siwan and Muzaffarpur are districts, and litchi belongs to the latter.

Hence, the answer is Muzaffarpur.