Step 1: Agriculture in Bihar.
Bihar is an important jute-producing state after West Bengal. The Purnea district is the leading producer of jute in Bihar.
Step 2: Analyzing the options.
(A) Gaya: Not a jute-producing district.
(B) Bhojpur: Not leading in jute.
(C) Purnea: Correct — largest jute producer in Bihar.
(D) Kaimur: Not significant in jute production.
Step 3: Conclusion.
The largest producer of jute in Bihar is Purnea.