Final answer:
Mayonnaise can be used as an emulsifier in dressings or spreads due to the presence of lecithins in egg yolk, which stabilize the emulsion of oil in vinegar.
Step-by-step explanation:
The ingredient among the listed options that can be used as an emulsifier in dressings or spreads is mayonnaise. An emulsion is a colloidal dispersion of one liquid in another, and it typically requires an emulsifying agent to remain stable. Mayonnaise is a classic example of an emulsion of oil in vinegar, stabilized by the lecithins present in egg yolk. The lecithins in the egg yolk allow it to interact with both polar substances (like vinegar) and nonpolar substances (like oil), thus preventing the ingredients from separating. Although components like vinegar, salt, and sugar all play roles in the flavor and preservation of dressings, they are not emulsifying agents.