ANS:A - kinetic energy.

At the boiling point of a liquid, it is in equilibrium with its vapor, which means the rate of vaporization equals the rate of condensation. At this point, the average kinetic energy of the molecules in both the liquid and vapor phases is equal because the temperature remains constant during boiling. Therefore, the correct choice is: kinetic energy Kinetic energy is the energy an object possesses due to its motion. In the context of molecules, such as those in a liquid or gas, kinetic energy refers to the energy associated with their movement. In a substance, molecules are in constant motion, vibrating, rotating, and moving around. This motion is random, with molecules moving in various directions and at different speeds. The kinetic energy of a molecule is directly related to its speed: the faster a molecule moves, the more kinetic energy it has. The kinetic energy (𝐾𝐸KE) of a molecule can be calculated using the formula: 𝐾𝐸=12𝑚𝑣2KE=21​mv2 Where:

• 𝑚m is the mass of the molecule,
• 𝑣v is its velocity.

In a system such as a liquid at its boiling point, the molecules possess a range of kinetic energies, with some moving faster than others. When a liquid is heated to its boiling point, the average kinetic energy of the molecules increases. At the boiling point, the molecules with enough kinetic energy can overcome the intermolecular forces holding them together and transition into the gas phase. In equilibrium at the boiling point, the average kinetic energy of the molecules in the liquid phase is equal to that of the molecules in the gas phase. This equilibrium is what defines the boiling point of a substance.