Substances which do not have free electrons in normal state, but free electrons can be obtained under special conditions like high temperature or addition of impurity, are called

The correct answer is semiconducting substances (option C). Semiconductors are materials that do not conduct electricity in their pure, undoped state because they lack free electrons. However, under special conditions, such as the application of high temperatures or the introduction of impurities (a process known as doping), these materials can gain free electrons and exhibit conductivity. This ability to control conductivity makes semiconductors fundamental to modern electronic devices. For instance, silicon, a widely used semiconductor, can be doped with elements like phosphorus or boron to create n-type or p-type semiconductors, respectively. These doped materials are essential in the creation of components like transistors, diodes, and integrated circuits, which are the building blocks of all electronic devices. The unique properties of semiconductors enable the precise control of electrical current, which is vital for the functioning of everything from computers to smartphones.