What type of fatty substance does not dissolve in water?

The correct answer is fats, which encompass a group of substances that include triglycerides, phospholipids, and sterols, among others. Fats are considered hydrophobic, meaning they do not dissolve in water. This characteristic is due to the molecular structure of fats, which consists primarily of long hydrocarbon chains. These nonpolar chains repel the polar molecules of water, resulting in fats not mixing with water.

In contrast, proteins consist of amino acids and can be either hydrophilic or hydrophobic depending on their structure. Carbohydrates, such as sugars and starches, are typically soluble in water due to their polar hydroxyl groups, which interact favorably with water molecules. Vitamins fall into two categories: water-soluble and fat-soluble, with the latter being soluble in fats but not in water. Thus, fats are uniquely defined by their inability to dissolve in water among the options presented.