Which of the following is an ingredient of polyester resin?

Maleic anhydride is indeed an essential ingredient in the formulation of polyester resin. Polyester resin is created through a chemical reaction between a polyol (usually a glycol) and an organic acid or anhydride, with maleic anhydride commonly being used due to its ability to form cross-linked structures that improve the physical properties of the resin. This cross-linking enhances the resin's strength, durability, and resistance to moisture and chemicals, making it suitable for various applications, particularly in the marine and automotive industries.

The other options, while relevant to composite materials and resins, do not directly contribute to the formulation of polyester resin. Polyethylene glycol is typically used in different types of applications, such as in lubricants or as a plasticizer, but is not a key ingredient in polyester resin. Epoxy is a different type of resin altogether, known for its superior adhesive properties and resilience but does not play a role in polyester resin formulation. Silicone, while useful as a sealant or in coatings, is not part of polyester resins either. Understanding the role of each material is crucial in the context of different resin systems and their applications.