A semiconductor is a material ________.

The correct choice identifies that a semiconductor typically has exactly four electrons in its outer orbit, which allows it to have unique electrical properties. Semiconductors, such as silicon and germanium, fall into this category because their atomic structure leads to a moderate level of conductivity that can be manipulated through the addition of impurities in a process called doping.

This property is what differentiates semiconductors from conductors (which typically have many free electrons and allow electric current to pass through easily) and insulators (which have very few free electrons and do not conduct electricity). The controlled conductivity of semiconductors is essential for the operation of electronic devices, such as diodes and transistors, which rely on the ability to switch between conducting and non-conducting states.

In summary, the defining characteristic of semiconductors is their four outer electrons, which facilitate their unique role in electronics, allowing for the control of electrical flow under certain conditions.