A hot-air balloon rises. Which is the best explanation for this observation?

Question

the difference in temperature between the air inside and outside the balloon produces convection currents

the cooler air outside the balloon pushes in on the walls of the balloon

the density inside the balloon is less than that of the surrounding air

the pressure on the walls of the balloon increases with increasing temperature

the rate of diffusion of cooler air is less than that of warmer air

Answer 1

The density inside the balloon is less than that of the surrounding air.