Assertion (A) : It is easier to pull a heavy object than to push it on a level ground and 

Reason (R) : The magnitude of frictional force depends on the nature of the two surfaces in contact. 

It is easier to pull a heavy object than to push it on a level ground. This is because the normal reaction in the case of pulling is less as compared by pushing.  $(f=\mu N)$. Therefore, the frictional force is small in case of pulling. 
The magnitude of frictional force depends on the nature of the two surfaces in contact. But is not the current explanation of statement – I .