From the law of atmosphere

Where
= constant and is number density where the height y = 0cm
= Number density at height y=3cm
Kb = Boltzmann constant 


Re-arranging the equation to have the value of the gravity,


Since it is 30% of value above surface, therefore 






Therefore the correct answer is C.
<span>The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.</span>