What allows plastic bumper covers and parts to be straightened with heat?

The correct answer is based on the concept of plastic memory, which refers to the ability of certain plastics to return to their original shape after being deformed when exposed to heat. Many types of plastics used in bumper covers are designed with this memory, allowing them to soften and become pliable when heated.

When heat is applied to a deformed plastic part, the material's temperature rises, allowing the polymer chains to move and relax, which helps in reshaping the part back to its original form. This property is particularly advantageous when repairing dents or scratches on bumper covers, as the technician can effectively manipulate the plastic to restore its appearance without needing to replace the part entirely.

Other options, while they have relevance in different contexts regarding plastics and heat, do not specifically address the reason why plastic bumper covers can be straightened using heat. For instance, heat retaining properties and thermal expansion focus on how materials respond to temperature changes but do not encapsulate the ability of plastics to revert to earlier shapes. Molecular bonding pertains more to how materials stick together at a molecular level, which does not directly relate to the physical shape recovery process in this scenario.