For what value of e is a collision perfectly elastic?

For what value of e is a collision perfectly elastic?
 
(a) 1
 
(b) 2
 
(c) 0.5
 
(d) 0
 
This question was addressed to me in an interview.
 
The origin of the question is Collision in section Work, Energy and Power of Physics – Class 11
Select the correct answer from above options 
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