A liquid is in equilibrium with its vapour at its boiling point. On the average, the molecules in the two phase have equal:

Kinetic energy

On the average, the molecules in the liquid and vapor phases at the boiling point have equal: (4) Kinetic energy.

At the boiling point, a liquid is in equilibrium with its vapor. The boiling point corresponds to the temperature at which the vapor pressure of the liquid equals the atmospheric pressure. In this equilibrium state, the liquid and vapor phases coexist and have similar kinetic energy.

Kinetic energy is related to the movement and speed of molecules. At the boiling point, molecules in the liquid phase have sufficient kinetic energy to overcome intermolecular forces and transition into the vapor phase. Similarly, molecules in the vapor phase have kinetic energy that allows them to move freely.

While options (1) intermolecular forces, (2) potential energy, and (3) temperature are important factors related to the behavior of molecules in different phases, they are not necessarily equal between the liquid and vapor phases at the boiling point.

Therefore, the correct answer is (4) Kinetic energy.