Which material becomes opaque after being heated and deformed?

Polycarbonate is known for its unique property of becoming opaque when heated and deformed. This behavior is primarily due to the physical changes it undergoes when subjected to high temperatures and mechanical stress. As polycarbonate is heated, its molecular structure may reorganize, leading to a loss of transparency, which is commonly observed when the material is processed or molded.

This effect is particularly important in applications where clarity is initially desirable, but certain processes, such as machining or thermal shaping, can induce this opacity. Polycarbonate's ability to withstand higher temperatures compared to other plastics allows it to be used in environments where heat may be a concern while still necessitating the potential for subsequent deformations.

Other materials listed may have different thermal and mechanical properties. For example, both polypropylene and polyvinyl chloride have good heat resistance but do not typically exhibit the same degree of opacity change as polycarbonate when processed at similar conditions. Acrylic, while clear and strong, tends to retain transparency during deformation processes unless extremely high temperatures are involved, leading to posts that might alter its clarity in different ways. Hence, polycarbonate stands out for its unique property of becoming opaque under the described conditions, making it the correct answer.