Final answer:
In thermoforming, thermoplastics are softened by a jet of hot gas, with the correct option being C) Thermoforming. Thermoforming allows reshaping of plastic sheets within molds after heating.
Step-by-step explanation:
In thermoforming, thermoplastic materials are softened by a jet of hot gas. The correct option is C) Thermoforming. Thermoforming is a process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. The heat causes the thermoplastic material to soften so it can be shaped, and once cooled, it becomes firm again. This characteristic allows thermoplastics to be remolded and recycled without negatively affecting the material's physical properties. In contrast, injection molding involves injecting molten material into a mold, while extrusion is the process of pushing a material through a die of a desired cross-section.