here C can identify the attributes , D, AB also , and You cannot identify only with E and F as E and F cannot derive AB or C or D...but they can derive B so if we have AE, AB they can derive all the attributes now ypu cannot add C,D to E,F as C,D are already candidate keys , if u add they are made into super keys.... so number of candidate keys are 5...
but they asked minimal candidate keys so 2 which are C,D