Question

what small amount of ingredient in cookies can enhance the overall flavor?
baking soda
eggs
sugar
vanilla

Explanation:

Vanilla is often used in small amounts in cookies. It is a flavoring agent that can enhance the overall taste of the cookies. Baking soda is a leavening agent, eggs act as binders and leavening agents in some cases, and sugar is a main sweetening ingredient (not typically used in a very small amount just for flavor - enhancing in the context of the question).

Answer:

Vanilla