Final answer:
Copper (Cu) is a solid at STP and a good conductor of electricity.
Step-by-step explanation:
Based on the information provided, the element that is a solid at standard temperature and pressure (STP) and a good conductor of electricity is Copper (Cu).
Copper is a metal that is shiny, solid, and a good conductor of heat and electricity. It is used in electrical wires due to its excellent electrical conductivity.
In contrast, the other options mentioned, Carbon (C), Oxygen (O), and Silicon (Si), are nonmetals which are typically not good conductors of electricity.