Which of the following statements about enthalpy is true?
enthalpy of a substance can be measured directly.
a chemical process must occur and then changes between the state of the reactants and the state of the products can be determined.
enthalpy is the gradual decline into disorder.
enthalpy is a measure of the change in pressure of a system.

   A chemical process must occur and then changes between the state of the reactants and the state of the products can be determined

Explanation:  Enthalpy represents the sum of the energy of the system with the product of the pressure and volume of that system. As a thermodynamic property, it expresses the ability to release heat from the system. In fact, enthalpy  tells us how much heat and work has changed during the chemical reaction under constant pressure. When measuring, measurements of the difference in enthalpy between the two states of the system is performed, before and after the chemical reaction, since total enthalpy can not be measured. This measurement of the enthalpy change can tell us, for example, whether the heat was released from the system during the reaction, or the system absorbed the heat.