Which substance is most likely to be a solid at standard temperature and pressure (STP)?

The correct choice is sodium (Na) because it is a metal and, at standard temperature and pressure (STP), most metals exist in a solid state. Sodium has a melting point of 97.8 °C, which is well above room temperature. Therefore, at STP, sodium will be in solid form.

In contrast, while water (H2O) is commonly found in solid form as ice, at STP it may exist as a liquid. Chlorine (Cl2) is a gaseous diatomic molecule at STP, and carbon dioxide (CO2) is typically a gas under these conditions, although it can exist as a solid (dry ice) at low temperatures and pressures. Thus, compared to these other substances, sodium is the most likely to be solid at STP.