Which of the following is a better nucleophile?

Electrophiles are

The decreasing order of nucleophilicity among the following nucleophiles is:
$(i) CH_3COO^-$
$(ii) CH_3O^-$
$(iii) CN^-$
$(iv) CH_3C_6H_4SO_3^-$

The products $(A)$ and $(B)$ in the following reaction are:

The cyanide ion $(CN^-)$ is:

$(a)$ Explain the following terms and provide the difference between them:
$(i)$ Reagent species
$(ii)$ Substrate species
$(b)$ $(i)$ What is the mechanism of a reaction?
$(ii)$ What are the uses of a reaction mechanism?
$(c)$ Depict the general reaction that occurs between a substrate and a reagent in an organic reaction.