The correct answer is C. Sugarcane.
Sugarcane is the main cash crop of Bihar state. It is grown in the districts of Muzaffarpur, Darbhanga, Samastipur, Begusarai, Vaishali, Saran, Patna, Nalanda, Lakhisarai, Sheikhpura, Bhagalpur, Munger, Khagaria, and Supaul.
Sugarcane is a tall, perennial grass that is grown for its sweet juice. The juice is extracted from the stalks and then processed to produce sugar, molasses, and ethanol. Sugarcane is a major source of income for farmers in Bihar. It is also a major source of employment in the state.
The other options are incorrect. Tea is not a major cash crop of Bihar. Rubber is grown in the southern states of India, such as Kerala and Tamil Nadu. Coconut and coffee are not grown in Bihar.