(a) : If the nucleophilic atom or the centre is same, nucleophilicity parallels basicity, i.e. more basic the species stronger is the nucleophile.
CH3O– > HO– > PhO– > AcO–
Here, the nucleophilic atom i.e. O is the same in all these species. This order can be easily explained on the general concept that a weaker acid has a stronger conjugate base.