Correct Answer: (B) $Mg^{2+}$
Detailed Solution:
Step 1: According to Pearson's Hard Soft Acid Base (HSAB) principle, "Hard" acids are typically characterized by small size, high positive charge, and low polarizability. "Soft" acids are larger, have lower positive charges, and are highly polarizable (usually heavy transition metals).
Step 2: $Cu^+$, $Cd^{2+}$, and $Hg^{2+}$ are heavy transition metal ions with large sizes and high polarizability. They are classified as Soft acids.
Step 3: $Mg^{2+}$ is an alkaline earth metal ion. It has a very small ionic radius, a relatively high charge density, and is non-polarizable. Therefore, it is a classic Hard acid.