Final answer:
The four-way valve is responsible for changing the flow of refrigerant vapor in a reverse-cycle system when switching from heating to cooling mode.
Step-by-step explanation:
In a reverse-cycle system, when switching from heating to cooling, the component responsible for changing the flow of refrigerant vapor is the four-way valve (B). This valve reverses the direction of the refrigerant, which allows the system to switch its operation mode. During the heating cycle, the evaporator absorbs heat from the outdoor air and the compressor raises the temperature and pressure of the gas, which then moves to the condenser where it releases heat into the room. Conversely, during the cooling cycle, the roles of the evaporator and condenser are reversed, and the four-way valve facilitates this change by altering the direction of the refrigerant flow.