Water efficiency of a crop refers to the amount of water required for its growth and development.

Among the options provided, Sugarcane requires the highest amount of water for its growth, making it the least water-efficient crop. It needs around 1800-2200 mm of water per season. On the other hand, Sunflower requires about 672.4 mm/season, Pearl Millet being a drought-tolerant crop requires around 350 mm/season, and Red gram uses about 250-400 mm/season of water.

Therefore, Sugarcane is the least water-efficient crop among the given options.