Introduction:
In chemical reactions, the direction of the reaction is determined by the changes in the thermodynamic properties of the system. These properties include entropy, enthalpy, and Gibbs energy. At constant pressure and temperature, the direction of the reaction is determined by the change in Gibbs energy.

Entropy:
Entropy (S) is a measure of the disorder or randomness of a system. In a chemical reaction, if the products have higher entropy than the reactants, the reaction tends to proceed in the forward direction. This is because an increase in entropy is favored as it leads to a more disordered system. However, if the products have lower entropy than the reactants, the reaction tends to proceed in the reverse direction.

Enthalpy:
Enthalpy (H) is a measure of the heat energy of a system. In a chemical reaction, if the products have lower enthalpy than the reactants, the reaction tends to proceed in the forward direction. This is because the reaction releases heat energy, making the system more stable. However, if the products have higher enthalpy than the reactants, the reaction tends to proceed in the reverse direction.

Gibbs Energy:
Gibbs energy (G) is a combination of entropy and enthalpy and represents the maximum amount of work that can be done by a system at constant pressure and temperature. The change in Gibbs energy (ΔG) determines the direction of a chemical reaction. If ΔG is negative, the reaction is spontaneous in the forward direction. If ΔG is positive, the reaction is non-spontaneous in the forward direction and tends to proceed in the reverse direction. If ΔG is zero, the reaction is at equilibrium.

Conclusion:
At constant pressure and temperature, the direction of a chemical reaction is determined by the change in Gibbs energy. If the change in Gibbs energy is negative, the reaction is spontaneous in the forward direction. Therefore, the correct answer is option 3: Gibbs energy.