Sodium benzoate is the correct answer for the substance used for the preservation of food grains. It is a commonly used food preservative that inhibits the growth of bacteria, yeasts, and molds. Sodium benzoate works by reducing the pH level of food, making it difficult for microorganisms to survive and multiply. It is particularly effective in preventing the growth of fungi and mold, which are common causes of spoilage in food grains. Vinegar, on the other hand, may have some preservative properties but is not commonly used for the preservation of food grains. Sodium chloride (common salt) is often used to enhance the taste of food but not for preservation. Potassium permanganate is a strong oxidizing agent and is not typically used for food preservation.