The mechanism of reaction (i) is \( S_2N_2 \), and the mechanism of reaction (ii) is \( SN_2 \). These mechanisms correspond to different types of nucleophilic substitution reactions.
Step 2: Conclusion.
The correct mechanism for reactions (i) and (ii) is \( S_2N_2 \) and \( SN_2 \), corresponding to option (b).