Among the following, which one is the least water-efficient crop?

It takes about 210 litres of water to produce 1 kg of sugarcane. The water footprint of sunflowers is 7-9 litres per kg. Millets are sown in dry areas with minimal irrigation. Red gram requires 35 to 40 cm of water during its entire growth period. Optimum moisture is required during budding, flowering, and pod formation stages. As Red gram is a rain-fed crop grown in assured rainfall areas, it usually does not need any irrigation.

Comparing these water requirements, sugarcane is significantly less water-efficient than the other options. Hence option (a) is the correct Answer.