Coefficient of restitution depends upon 
 

The dependence is on the substance of the objects in contact. 

If the bodies totally restore their original shapes after colliding, then its restitution coefficient is 1.

If one of the bodies colliding loses some of its form, then 0<e<1 )

If the objects colliding entirely lose their form, then (e=0).

Hence the correct answer is the materials of the colliding bodies.