Answer:
There are 12 oxygen atoms in 8C12O.
The expression for the frictional force between the sled and the ground is:

where

is the coefficient of friction, m is the mass of the object and

is the gravitational acceleration.
The friction force in our problem is F=80.85 N. The mass of the object is m=15 kg. Re-arranging the formula, we can find the value of k:
The most accurate answer among the choices would be the fourth one, because if the weight of the air displaced is greater than the balloon's weight, the balloon will float upwards. Density of the air also plays a part. Hot air = less dense. Hope my answer has come to your help.