The decreasing order of the rate of the above reaction with nucleophiles (Nu^-) a to d

(a) PhO^-    

(b) AcO^-

(c) OH^-

(d) CH₃O^-

d > c > b > a

The order of nucleophilicity of the oxyanions is as follows:

(d) CH₃O^- > (c) OH^- > (b) AcO^- > (a) PhO^-

This order can be explained by the following factors:

Size: The smaller the nucleophile, the more easily it can approach the electrophile and attack it. CH3O- is the smallest oxyanion, followed by OH^-, AcO^-, and PhO^-.

Charge: The more negative the charge on a nucleophile, the more nucleophilic it is. All of the oxyanions have a negative charge, but CH₃O^- has the most negative charge.

Electronic effects: The presence of electron-withdrawing groups (such as the carbonyl group in AcO^-) decreases the nucleophilicity of a molecule. PhO^- also has an electron-withdrawing group, but it is not as strong as the carbonyl group.

Therefore, the order of nucleophilicity of the oxyanions is (d) CH₃O^- > (c) OH^- > (b) AcO^- > (a) PhO^-. This means that the reaction rate will be fastest with CH₃O^-, followed by OH^-, AcO^-, and PhO^- in decreasing order of reactivity.