Answer:
convection
Step-by-step explanation:
Convection is a mode of heat transfer in which the particles of a substance actually move from place to place.
During convection, warmer and less dense air rises while colder and denser air sinks downwards under gravity.
Hence; when hot air balloon rises because the fire warms up the air and the air circulates to fill the balloon and lift it at the same time, convection has taken place.