What defines thermosetting materials?

Thermosetting materials are defined by their unique property of becoming hard and rigid after they have been set or cured through a chemical process, typically involving heat and/or a curing agent. Once these materials have undergone this setting process, they cannot be remoulded or reshaped even with the application of heat. This characteristic is due to the formation of a three-dimensional cross-linked structure within the polymer, which provides strength and stability.

When comparing this to the other options, the property that thermosetting materials cannot be reshaped after setting is the definitive characteristic that distinguishes them from thermoplastics, which can be reformed multiple times. Additionally, thermosetting materials are generally not soluble in water and their melting points vary, but this is not a defining feature. Instead, the key focus is on the irreversible setting process that characterizes these materials, further solidifying option B as the correct answer.