Question

two balls of same size one of rubber and second of iron are placed on floor . which one has more inertia?

Answer 1

Since the  sizes are the same ,but we know that the  iron is more denser than rubber .Also, as the density is proportional to mass, the mass of iron is more than the mass of rubber 
By the newton law of motion , inertia is directly proportional to the mass so the iron ball will possess more inertia for being rest .
So, in the result the iron ball will find it difficult to move as compared to the rubber ball which will more easily.

Answer 2

Given two balls of same size one of rubber and second ball of iron are placed on floor.         
                     Iron ball has more Inertia .

Reason :
 Inertia is a natural tendency of every object to resit the change in state of rest or  uniform motion.
Inertia is a measure of mass.
As mass increases, inertia also increases.

So here iron is more denser than rubber ball so its mass is also more .
Hence Iron ball possess  more inertia.