Introduction:
In order to determine the most stable compound among Li2O, MgO, Cs2O, and KI, we need to consider several factors. Stability is influenced by various factors such as ionic size, electronegativity, lattice energy, and the stability of the resulting ions.

Factors influencing stability:
1. Ionic size: Smaller ions tend to form more stable compounds due to stronger electrostatic forces of attraction between the ions. Larger ions experience weaker forces of attraction, leading to lower stability.

2. Electronegativity: Electronegativity difference between two atoms influences the polarity of the bond. Higher electronegativity difference leads to a more polar bond, which increases the stability of the compound.

3. Lattice energy: Lattice energy is the energy released when gaseous ions combine to form a solid compound. Higher lattice energy indicates greater stability.

Analysis of compounds:
Now, let's analyze the given compounds based on the above factors to determine their relative stability.

1. Li2O:
- Lithium (Li) is a small cation, and oxygen (O) is a small anion.
- The electronegativity difference between Li and O is significant, resulting in a highly polar bond.
- The lattice energy for Li2O is relatively high due to the strong attraction between the ions.
- Li2O is expected to be a stable compound.

2. MgO:
- Magnesium (Mg) is a larger cation compared to Li, and oxygen (O) is a small anion.
- The electronegativity difference between Mg and O is moderate, resulting in a relatively polar bond.
- The lattice energy for MgO is high, but not as high as Li2O due to the larger size of Mg cations.
- MgO is stable but less stable than Li2O.

3. Cs2O:
- Cesium (Cs) is a larger cation compared to Li and Mg, and oxygen (O) is a small anion.
- The electronegativity difference between Cs and O is relatively low, resulting in a less polar bond.
- The lattice energy for Cs2O is lower due to the larger size of Cs cations.
- Cs2O is less stable compared to Li2O and MgO.

4. KI:
- Potassium (K) is a smaller cation compared to Cs, and iodine (I) is a larger anion compared to O.
- The electronegativity difference between K and I is moderate, resulting in a relatively polar bond.
- The lattice energy for KI is lower compared to the oxides due to the larger size of I anions.
- KI is expected to be less stable than Li2O and MgO, but more stable than Cs2O.

Conclusion:
Based on the analysis, the most stable compound among the given options is Li2O. It has a smaller cation, a highly polar bond, and a higher lattice energy compared to the other compounds. MgO is less stable than Li2O, followed by KI, and Cs2O is the least stable compound.

It is an ionic bond b\w small oxygen and  large Mg