A* algorithm is guaranteed to find an optimal solution if

UGC NET December 2012 computer science

A* algorithm is guaranteed to find an optimal solution if
a) h' is always 0
b) g is always 1
c) h' never overestimates h
d) h' never underestimates h

Answer is (C)h' never overestimates h 
(96% confirmed, not fully sure)