The correct answer is indigo.
Indigo is a blue dye that was produced in large quantities in Bihar during the colonial period. The indigo plant was grown on large plantations, and the dye was extracted from the leaves. Indigo was a valuable commodity, and it was exported to Europe and other parts of the world.
Sugar was also produced in Bihar during the colonial period, but not on the same scale as indigo. Sugarcane was grown on smaller plantations, and the sugar was processed in mills. Sugar was a less valuable commodity than indigo, and it was mostly consumed in India.
Cotton was also grown in Bihar during the colonial period, but it was not a major crop. Cotton was grown on small farms, and the cotton was used to make cloth. Cotton was not a valuable commodity, and it was mostly consumed in India.
Opium was not produced in Bihar during the colonial period. Opium was grown in other parts of India, such as Bengal and Assam. Opium was a valuable commodity, and it was exported to China.
Indigo was the most important commodity produced in Bihar during the colonial period. It was a valuable commodity, and it was exported to Europe and other parts of the world.