The correct answer is: Soap.
Soap is a product of the saponification of fats or oils with alkalis. The saponification reaction is an exothermic reaction, meaning that it releases heat. This heat can be used to generate steam, which can then be used to power machinery in the soap manufacturing process. As a result, soap production requires relatively little energy compared to other industries.
Sugar is produced by extracting sucrose from sugarcane or sugar beets. The extraction process is energy-intensive, as it requires the use of heat and pressure. Sugar refining is also energy-intensive, as it involves the removal of impurities from the sucrose solution. As a result, sugar production requires a significant amount of energy.
Vegetable oil is produced by extracting oil from seeds or fruits. The extraction process is energy-intensive, as it requires the use of heat and pressure. Vegetable oil refining is also energy-intensive, as it involves the removal of impurities from the oil. As a result, vegetable oil production requires a significant amount of energy.
Caustic soda is produced by the electrolysis of sodium chloride. The electrolysis process is energy-intensive, as it requires the use of electricity. Caustic soda production also requires the use of heat, as the sodium chloride must be melted before it can be electrolyzed. As a result, caustic soda production requires a significant amount of energy.
In conclusion, soap production requires the least amount of energy per tonne of product.