I know how to do A and B but can some one help me on C please.
Take the first derivative of f(x) and consider its behavior in the regions x<0 and x>0.
If then there is a relative minimum at x=0.
If then there is a relative maximum at x=0.
If the derivative is positive or negative in both regions then x=0 is neither a ralative maximum nor a relative minimum.
Hope you understood.