There are some plastics that when moulded once cannot be softened by heating. These are called:

thermosetting polymers

The correct answer is option 1. thermosetting polymers.

Thermosetting polymers, also known as thermosets, are a type of plastic that, once molded and cured, cannot be softened or reshaped by heating. This is because thermosetting polymers undergo irreversible chemical reactions during the curing process, forming a three-dimensional network of cross-linked polymer chains. Once this network is formed, applying heat will not cause the material to soften or melt.

On the other hand, thermoplastic polymers (such as polyvinyl chloride, thermoplastic polyurethanes, and high-density polyethylene) can be softened by heating and reshaped multiple times because they do not undergo irreversible chemical reactions upon heating. Instead, they soften when heated and solidify when cooled, allowing for reshaping.