A catalyst is a substance which:

Shortens the time to each equilibrium

The correct answer is option 4. Shortens the time to each equilibrium.

A catalyst is a substance that speeds up a chemical reaction without being consumed in the process. It achieves this by providing an alternative reaction pathway with a lower activation energy. Let's analyze each option to see which correctly describes the role of a catalyst:

1. Supplies energy to the reaction:

This statement is incorrect. A catalyst does not supply energy to the reaction. Instead, it lowers the activation energy required for the reaction to proceed.

2. Increases the equilibrium concentration of the product:

This statement is incorrect. A catalyst does not change the equilibrium concentrations of reactants and products. It only helps the system reach equilibrium faster.

3. Changes the equilibrium constant of the reaction:

This statement is incorrect. A catalyst does not change the equilibrium constant (\( K \)) of a reaction. The equilibrium constant is determined by the thermodynamics of the reaction, and a catalyst only affects the rate at which equilibrium is reached.

4. Shortens the time to reach equilibrium:

This statement is correct. A catalyst increases the rate of both the forward and reverse reactions equally, thereby allowing the system to reach equilibrium faster without affecting the position of the equilibrium.

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