Addition of catalyst during a chemical reaction alters:

Explanation

Option C, "Ea" (Activation Energy), is the correct answer. A catalyst is a substance that speeds up a chemical reaction by providing an alternative pathway with lower activation energy. It does not alter the enthalpy (H) of the reaction (Option A), the energy (E) of the reactants or products (Option B), or the collision frequency (Option D). Activation energy is the minimum energy required to start a chemical reaction. By providing an alternate pathway with lower activation energy, a catalyst allows the reaction to occur at a faster rate. This is why option C is the correct choice.