The law which states that the amount of heat evolved or absorbed in a process is the same whether the reaction is completed in one or several steps is called:

Explanation

Hess's Law is a fundamental principle in thermochemistry that states the enthalpy change of a chemical reaction is the same regardless of whether it occurs in a single step or multiple steps. This law is based on the principle that enthalpy is a state function, meaning it depends only on the initial and final states of the reaction, not the pathway taken. The First Law of Thermodynamics pertains to the conservation of energy and does not specifically address the stepwise nature of reactions. The term 'Law of conservation' is too broad and does not specifically apply to this context. Therefore, the correct answer is Hess's Law, as it directly addresses the question concerning heat changes in reactions.