Answer: C. Mg²⁺ has a greater charge and smaller radius than Na⁺, so it attracts delocalized electrons more strongly. Stronger metallic bonding requires more energy to overcome during melting, so Mg has the higher melting point.

A: treats the larger radius of Na⁺ as strengthening the attraction and carries that reversal through to melting point.
B: identifies Mg as more strongly bonded but reverses the relationship between bonding strength and melting point.
D: assigns the higher melting point to Mg but incorrectly assigns stronger metallic bonding to Na.

Remember: greater cation charge and smaller radius strengthen metallic bonding; stronger bonding generally corresponds to a higher melting point.