The correct option is B Charles' law
Hot air is used in sports and meterological observations because, hot air has lesser density,
since density is inversely proportional to volume.
The principle used here is based on Charle's law.
According to Charle's law,
V∝T at constant pressure
Greater the temperature, greater the volume and greater the volume, lesser the density.
(Since the volume is inversely proportional to the density.)