Question

Which situations describe an elastic collision?

Answer 1

Hi !

In elastic collision e (which is coefficient of restitution) is equal to 1 i.e., 

e = 1

So, whenever in the question it is given that e = 1 , it implies elastic collision and vice-versa.

Hope this may help you!

Answer 2

It seems like you have forgotten to provide the options.
An elastic collision is one where the magnitude of velocity before collision doesn't change after the collision. Or the coefficient of restitution(e) is 1.