Name a common emulsifier used in salad dressings.

Egg yolk is a common emulsifier used in salad dressings due to its ability to stabilize mixtures of oil and water-based ingredients. It contains lecithin, a phospholipid that allows oil and water to blend smoothly, preventing separation and creating a creamy texture. This property makes egg yolk especially valuable in classic dressings such as mayonnaise and Caesar dressing. When egg yolk is whisked with oil, the lecithin molecules help to surround and disperse the oil droplets within the water phase, resulting in a stable emulsion.

While mustard can also act as a mild emulsifier due to its own emulsifying properties, and vegetable oil is a primary ingredient for richness in dressings, it does not serve as an emulsifier itself. Vinegar, although an essential component of salad dressings for acidity and flavor, does not have emulsifying properties and merely acts as a flavoring agent. Thus, egg yolk's unique ability to form stable emulsions is what makes it the correct answer in this context.