A catalyst is a substance which:

Shortens the time to each equilibrium

A catalyst is a substance that increases the rate of a chemical reaction without being consumed or permanently altered in the process. It achieves this by providing an alternative reaction pathway with lower activation energy. By lowering the activation energy, a catalyst facilitates the breaking and forming of chemical bonds, allowing the reaction to proceed more rapidly.

Importantly, a catalyst does not supply energy to the reaction (1). It does not change the equilibrium concentration of the product (b=2) nor the equilibrium constant of the reaction (3). A catalyst affects the kinetics of the reaction, specifically the rate at which the reaction reaches equilibrium, but it does not alter the position of the equilibrium itself.

By reducing the activation energy, a catalyst allows more reactant molecules to overcome the energy barrier and reach the transition state, leading to a faster rate of reaction. As a result, the time required for the reaction to reach equilibrium is shortened, but the equilibrium concentrations and the equilibrium constant remain unchanged.

Therefore, the correct answer is (4) Shortens the time to reach equilibrium.