Which of the following is a thermoforming polymer?

Polystyrene is classified as a thermoforming polymer because it can be easily heated and molded into various shapes without undergoing a chemical change. Thermoforming polymers are distinguished by their ability to soften when heated. Once shaped, they can be cooled and retain their new form, making them suitable for applications such as packaging, containers, and other lightweight products.

In contrast, epoxy resin is a thermosetting polymer, meaning that once it is cured through a chemical process, it cannot be remolded or reshaped. Steel and aluminum, being metals, do not belong to the category of polymers at all, as they are defined by their metallic bonding and properties rather than the molecular structure associated with polymers. Therefore, polystyrene stands out as the correct answer due to its thermoplastic nature, allowing for reshaping through heat.