Answer and Explanation:
The gravitational acceleration 'g' depend directly on the mass of the object or body and inversely on the distance or radius squared:

where
= mass of the object
G = Gravitational constt
Thus the plastic ball is lighter and have low mass as compared to the steel and golf balls.
This is the reason that a plastic ball have a low value of acceleration as compared to that of steel and golf balls with higher values of acceleration.