Which of the following methods is least effective for removing grease stains from upholstery?

The least effective method for removing grease stains from upholstery is using powder additives. Powder additives typically work by absorbing surface soil and may not be able to penetrate through greasy stains effectively. Grease requires a liquid or solvent that can dissolve the oily compounds, and powders do not provide that necessary solubility action.

In contrast, wet cleaning involves the use of water, detergents, or solvents that can effectively break down and remove grease. Spot cleaning with solvent allows for targeted treatment of the stain and utilizes the solvent's properties to dissolve the grease. Dry cleaning, while not water-based, employs solvents specifically designed to dissolve oils and grease, making it effective for such stains. Consequently, while powder additives might be useful in removing dry soil, they lack the efficacy needed for dealing with greasy substances, thus making them the least effective option in this context.