Electronic configurations and types of elements

Correct answer: A. Because it is a metalloid and semiconductor

The governing concept is the relationship between the classification of an element and its electrical behavior. Silicon is a metalloid, so its conductivity lies between that of a good conductor and an insulator. More importantly, its conductivity can be controlled by temperature, light, electric fields, and deliberate addition of small amounts of impurities, a process called doping. This controllability allows silicon to form diodes, transistors, integrated circuits, and solar cells. It is a solid element, not a gas, salt, or colorless liquid. Therefore option A is correct; the other options do not explain its electronic usefulness.

Correct answer: A. Copper is a metal, silicon is a metalloid, and oxygen is a non-metal

Copper is a metal because it is lustrous, malleable, ductile, and a good conductor of heat and electricity. Silicon is commonly classified as a metalloid because it shows intermediate properties: it is a semiconductor and has characteristics between metals and non-metals. Oxygen is a non-metal; it is a gaseous element and is not lustrous, malleable, or electrically conducting under ordinary conditions. Therefore option A gives the correct classification of all three substances. The other options incorrectly call copper a non-metal or compound, silicon a gas or liquid rather than a solid metalloid, and oxygen a metal, mixture, or alloy. Classification is based on elemental identity and characteristic properties.