An extensive property is a property that depends on the size or amount of a system.

Density: Density is defined as the mass per unit volume of a substance. It is an intensive property because it does not depend on the size or amount of the substance. For example, the density of a substance remains the same whether we have a small or large amount of it.

Force: Force is a physical quantity that is measured in Newton (N). It is a vector quantity that describes the interaction between two objects. Force is not an extensive property because it does not depend on the size or amount of the system. Instead, it depends on the mass and acceleration of the objects involved.

Thermal Conductivity: Thermal conductivity is a measure of a material's ability to conduct heat. It is an intensive property because it does not depend on the size or amount of the material. For example, the thermal conductivity of a material remains the same whether we have a small or large amount of it.

Internal Energy: Internal energy is the total energy stored in a system. It includes the kinetic and potential energies of the particles within the system. Internal energy is an extensive property because it depends on the size or amount of the system. If we have more particles or more mass, the internal energy of the system will be higher.

Conclusion: Among the given options, the only extensive property is the internal energy. Density, force, and thermal conductivity are all intensive properties because they do not depend on the size or amount of the system.