Explanation

Water-use efficiency (WUE) measures the economic agricultural yield produced per unit of water consumed (evapotranspiration):

– Sugarcane (Least water-efficient): A perennial, heavy water-guzzling commercial crop requiring 1800 to 2200 mm of water per growing season. Its massive physiological biomass and long vegetative cycle demand enormous irrigation volume relative to dry-matter yield.
– Sunflower: Moderately water-efficient oilseed crop requiring approximately 600 to 700 mm per season.
– Pearl millet (Bajra): A highly drought-tolerant C4 coarse cereal requiring only 300 to 350 mm per season, exhibiting superior water productivity in arid terrains.
– Red gram (Arhar/Tur): A drought-hardy pulse crop with a deep taproot system requiring roughly 250 to 400 mm per season.

Therefore, the least water-efficient crop is Sugarcane — Option A.