The phase change process that is taking place when a substance is in equilibrium between liquid and gas phases is evaporation and condensation1. Evaporation is the process of liquid molecules escaping into the gas phase, while condensation is the process of gas molecules returning to the liquid phase.
When a substance is in a closed container at the boiling point, then the liquid is boiling and the gas is condensing at the same rate without net change in their relative amount2. This means that the liquid and gas phases are in equilibrium at the boiling temperature.
The other options you listed are not correct for this situation. Melting and boiling are phase changes between solid and liquid, and liquid and gas, respectively, but they are not in equilibrium. Expanding and contracting are changes in volume due to changes in temperature or pressure, but they are not phase changes. Freezing and thawing are phase changes between liquid and solid, but they are not in equilibrium.