6. A balloon inflated with helium is able to float toward the ceiling because a) the weight of the balloon is greater than the weight of the air displaced by the balloon.
b) the density of the balloon is less than the density of the atmosphere.
c) The kinetic energy inside the balloon is less than the kinetic energy of the atmosphere
d) the pressure of the gas inside the balloon is greater than the atmospheric pressure.

b) the density of the balloon is less than the density of the atmosphere.

Explanation:

We must take into account that density is defined as the relationship between mass and volume.

The helium density is 0,1785 [kg/m3]

The air density is 1.2 [kg/m3]

Therefore a substance with lower density (helium) will try to rise on substances with heavier density (air).

An example of this is when we mix water with oil, the water is heavier than the oil, that is, the density of the water is greater than the density of the oil, therefore the oil will be located on the surface of the water and not at the bottom.

Another example is that of a balloon inflated with air is common, this balloon doesn't rise because the air density outside the balloon equals the air density inside the balloon, the balloon will fall to the ground since it contains the weight of the air contains the weight of the material of the balloon.