The Answer:
The correct answer is option 'A', Hess's law.

Explanation:
Hess's law states that the net heat change in a chemical reaction is the same whether it is brought about in two or more different ways in one or several steps. In other words, the enthalpy change of a reaction is independent of the pathway taken.

Key Points:
Here are some key points to understand why Hess's law holds true:

1. Enthalpy: Enthalpy is a thermodynamic property that represents the heat content of a system. It is denoted by the symbol 'H'. Enthalpy change, ΔH, is the difference in enthalpy between the reactants and products of a chemical reaction.

2. Pathway Independence: According to Hess's law, the enthalpy change of a reaction is independent of the pathway taken. It means that the overall heat change in a reaction is determined only by the initial and final states of the reactants and products, regardless of the intermediate steps involved.

3. Enthalpy Diagrams: Enthalpy diagrams are graphical representations that show the energy changes during a chemical reaction. Hess's law can be visualized using these diagrams. Since the enthalpy change is pathway independent, the overall enthalpy change can be calculated by summing up the enthalpy changes of individual steps or reactions.

4. Enthalpy of Formation: Hess's law is particularly useful when calculating the enthalpy of formation of a compound. The enthalpy of formation is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. By using known enthalpy values for other reactions, the enthalpy of formation of a compound can be determined indirectly.

5. Experimental Validation: Hess's law has been experimentally validated by measuring the enthalpy changes of various reactions and comparing them with theoretically calculated values. Consistent results have confirmed the validity of Hess's law.

Conclusion:
Hess's law is a fundamental principle in thermodynamics that allows us to calculate the heat change in a chemical reaction by considering the enthalpy changes of individual steps or reactions. It provides a valuable tool for understanding and predicting the energy changes associated with chemical reactions.