Which of the following can be said about thermosetting plastics?

Thermosetting plastics are indeed characterized by their ability to harden permanently after being set. This process occurs during the curing phase, where heat and/or chemical processes are applied to shape the material. Once thermosetting plastics are hardened, they cannot be reshaped or remolded, which is a defining property of these materials.

As for the other options, reshaping thermosetting plastics is not possible due to the irreversible chemical bonds that form during the curing process. While they may be pliable when heated during the initial processing stage, they do not remain flexible in the same way that thermoplastics do. Additionally, while they are sometimes used for insulation purposes due to their heat resistance and dielectric properties, this is not an inherent characteristic that defines thermosetting plastics across the board. Therefore, the most accurate statement about thermosetting plastics from the choices provided is that they harden permanently after being set.