The correct option is B I<III<II<IV
Melting point∝ ionic nature of halide
We know that,
Greater the electronegativity difference between the cation and the anion in a compound, higher will be the ionic character.
The electronegativities of Na, Cu, Ag, Cl and I are 0.93, 1.90, 1.93, 3.16 and 2.66 respectively.
The electronegativity difference in NaCl = 3.16−0.93 = 2.23
The electronegativity difference in NaI = 2.66−0.93 = 1.73
The electronegativity difference in CuI = 2.66−1.90 = 0.76
The electronegativity difference in AgI = 2.66−1.93 = 0.73
From the above calculations we can see that the ionic character decreases in the following order: NaCl>NaI>CuI>AgI
∴ The increasing order of melting point is AgI5580C<CuI6060C<NaI6610C<NaCl8010C