Final answer:
The substance expected to have the lowest enthalpy at 298K is solid sodium chloride (NaCl).
Step-by-step explanation:
The substance expected to have the lowest enthalpy at 298K is c) Solid sodium chloride (NaCl). Enthalpy is the heat energy that is released or absorbed during a chemical reaction or physical change. In this case, we can compare the enthalpy changes involved in the phase changes of the substances given.
When a substance changes from solid to liquid or from liquid to gas, energy is absorbed and the enthalpy increases. The enthalpy of vaporization (the amount of heat required to convert a substance from liquid to gas at its boiling point) is usually higher than the enthalpy of fusion (the amount of heat required to convert a substance from solid to liquid at its melting point).
In the given options, NaCl is a solid at 298K, so its enthalpy would be lower than substances that exist as gases at that temperature.