Which of the following substance has the highest melting point ?

(A) BaO

(B) MgO

(C) KCl

(D) NaCl

To solve this question, let's evaluate each compound based on the factors that affect melting point, particularly lattice energy. Lattice energy is higher for compounds with smaller ions and greater ionic charges, leading to stronger ionic bonds and higher melting points.

### Factors affecting melting point:
1. **Ionic size**: Smaller ions lead to higher lattice energy due to the ions being closer together in the lattice.
2. **Ionic charge**: Higher charges increase the electrostatic forces between the ions, leading to higher lattice energy.

Let’s examine each compound:
- **BaO**: Barium oxide consists of \( \text{Ba}^{2+} \) and \( \text{O}^{2-} \). The ionic charge is high (+2 and -2).
- **MgO**: Magnesium oxide consists of \( \text{Mg}^{2+} \) and \( \text{O}^{2-} \). It also has a high ionic charge (+2 and -2), but magnesium ions are smaller than barium ions.
- **KCl**: Potassium chloride consists of \( \text{K}^{+} \) and \( \text{Cl}^{-} \). The ionic charge is lower (+1 and -1), and potassium ions are larger.
- **NaCl**: Sodium chloride consists of \( \text{Na}^{+} \) and \( \text{Cl}^{-} \). Like KCl, it has a lower ionic charge (+1 and -1), but sodium ions are smaller than potassium ions.

### Comparison:
- **MgO** has the smallest cation (Mg²⁺) and the highest ionic charge (2+ and 2-). This results in the strongest electrostatic attraction and, therefore, the highest lattice energy among the given compounds.
- **BaO** has the same charge as MgO but a larger cation (Ba²⁺), leading to lower lattice energy.
- **KCl** and **NaCl** have lower charges (+1, -1) compared to oxides (+2, -2), and their lattice energies are thus lower.

### Conclusion:
Magnesium oxide (MgO) has the highest melting point due to its high ionic charge and smaller ionic size.

**Answer: (B) MgO**