Final answer:
A reaction with positive enthalpy is an endothermic reaction where heat is absorbed from the surroundings, making energy act as a reactant and increasing the system's enthalpy.
Step-by-step explanation:
A reaction with a positive enthalpy change (ΔH) is known as an endothermic reaction. This means that heat is required for the reaction to occur as it is absorbed from the surroundings into the system. In other words, energy is acting as a reactant, which leads to an increase in the enthalpy of the system.
A common misconception is that a reaction with positive enthalpy will never be spontaneous (option D). However, spontaneity is not only dictated by enthalpy changes but also by entropy changes and temperature, as described by Gibbs free energy. Positive enthalpy does not necessarily equate to a large activation energy; the activation energy is the energy required to reach the transition state, which could be large or small regardless of whether the reaction is endothermic or exothermic.
Therefore, the correct answer to the question about a reaction with positive enthalpy is that heat is required to make the reaction occur (Option C).