“Prerogative” is perfect (5)
I believe the answer is:
right
I believe this is a double definition.
'prerogative' is the first definition.
(a prerogative is a legal right)
'perfect' is the second definition.
(thesaurus)
'is' is the link.
(Other definitions for right that I've seen before include "Complete" , "Correct - claim allowed" , "ideal" , "Morally defensible" , "Prerogative" .)