Which acid is NOT an ingredient of polyester resin?

In the context of polyester resin formulation, citric acid is not a typical ingredient. Polyester resins are primarily created through the reaction of unsaturated acids, such as maleic anhydride or fumaric acid, with glycols. Unsaturated acids are crucial for the resin to cure effectively under heat or the presence of a catalyst.

Saturated acids, while they may be involved in some other chemical processes, are not primary components of polyester resin. Glycol serves as the backbone in tandem with the acid to create the polyester chain. Therefore, citric acid, being a different kind of organic acid, does not play a role in standard polyester resin production, making it the correct answer to the question.