The correct answer is option 'D': Ethanol.

Preserving food is an ancient practice that has been used throughout history to extend the shelf life of various food items and prevent spoilage. There are several methods of food preservation, including drying, canning, freezing, and fermentation. Many of these methods involve the use of certain ingredients or processes to inhibit the growth of microorganisms that cause food spoilage.

Let's explore the options given in the question and understand why option 'D' is the correct answer:

a) Vinegar:
- Vinegar is a commonly used ingredient for food preservation.
- It is acidic in nature and contains acetic acid, which has antimicrobial properties.
- The acetic acid in vinegar inhibits the growth of bacteria and other microorganisms, thereby preserving the food.

b) Salt and sugar:
- Salt and sugar are both commonly used ingredients for food preservation.
- They work by creating an environment with low water activity, which inhibits the growth of microorganisms.
- Salt is particularly effective against bacteria, while sugar is more effective against yeasts and molds.

c) Sugar and vinegar:
- Both sugar and vinegar are commonly used ingredients for food preservation.
- Sugar acts as a preservative by creating a high osmotic pressure environment, which inhibits the growth of microorganisms.
- Vinegar, as mentioned earlier, contains acetic acid, which has antimicrobial properties.

d) Ethanol:
- Ethanol, or ethyl alcohol, is not commonly used for preserving food.
- While ethanol can be used as a preservative in certain products like alcoholic beverages, it is not as effective in preserving other types of food.
- Ethanol's antimicrobial properties are not as potent as those of vinegar or other ingredients commonly used for food preservation.

In conclusion, the correct answer is option 'D': Ethanol. While vinegar, salt and sugar, and sugar and vinegar are all commonly used ingredients in food preservation, ethanol is not as commonly used for this purpose.