For a surface molecule
a) the net force on it is zero
b) there is a net downward force
c) the potential energy is less than that of a molecule inside
d) the potential energy is more than that of a molecule inside
The answer is the option (b) and (d)
The force exerted on a molecule by all other neighbouring molecules is equal and opposite in all directions. Hence the horizontal component is zero.
A downward force acts vertically, which is not cancelled and hence b is the correct option.
The potential energy of the molecules on the surface is negative as compared to the molecules in the subsequent layers. But the magnitude of this PE is higher than the below molecules. Hence option d is also correct.