Which is the oldest and largest canal irrigation system of Bihar?

The correct answer is C, the Sone canal system. Work on the Sone began in the colonial period with an anicut across the river at Dehri in Rohtas district, and the system has been extended ever since, the Indrapuri barrage replacing the old anicut in 1968. Its eastern and western main canals together command the largest area of any canal network in the state and turned the Sone plain into the rice growing heart of south Bihar.

Option A, the Kosi system, came much later, after the agreement with Nepal of 1954, and its canals serve the north east. Option B, the Gandak system, also belongs to the period after independence and serves the north west. Option D, a Bagmati canal system, exists only as small schemes, since the Bagmati is chiefly a flood problem rather than an irrigation source. Date the three systems in order, Sone first, then Kosi and Gandak, and such questions solve themselves.