The correct answer is B. Tubewells.
Canals are a major source of irrigation in many parts of India, but in Bihar, tubewells are the most important source. This is because Bihar has a relatively dry climate and there is not enough water in the rivers and streams to support large-scale canal irrigation. Tubewells are able to tap into groundwater, which is a more reliable source of water in Bihar.
Wells are also a source of irrigation in Bihar, but they are not as important as tubewells. This is because wells are often not deep enough to reach the groundwater, and they can also be contaminated with pollutants.
Ponds are not a major source of irrigation in Bihar. This is because ponds are often too small to irrigate large areas of land, and they can also be dry during the summer months.
In conclusion, the major source of irrigation in Bihar is tubewells. This is because Bihar has a relatively dry climate and there is not enough water in the rivers and streams to support large-scale canal irrigation. Tubewells are able to tap into groundwater, which is a more reliable source of water in Bihar.