NaCl bcoz as the size of cation increase the ionic bond becomes weaker. So NaCl

Introduction:
In order to determine which compound is the most ionic among NaCl, KCl, RbCl, and CsCl, we need to consider the factors that contribute to ionic character in a compound. Ionic character is influenced by the electronegativity difference between the elements involved in the bond formation.

Electronegativity:
Electronegativity is a measure of an atom's ability to attract electrons towards itself in a chemical bond. Generally, the larger the electronegativity difference between two atoms, the more ionic the bond will be.

Analysis:
In this case, we are comparing the electronegativity differences between Na and Cl, K and Cl, Rb and Cl, and Cs and Cl.

- Sodium (Na) has an electronegativity value of 0.93, while Chlorine (Cl) has an electronegativity value of 3.16. The electronegativity difference is 2.23.
- Potassium (K) has an electronegativity value of 0.82, while Chlorine (Cl) has an electronegativity value of 3.16. The electronegativity difference is 2.34.
- Rubidium (Rb) has an electronegativity value of 0.82, while Chlorine (Cl) has an electronegativity value of 3.16. The electronegativity difference is 2.34.
- Cesium (Cs) has an electronegativity value of 0.79, while Chlorine (Cl) has an electronegativity value of 3.16. The electronegativity difference is 2.37.

Conclusion:
Among NaCl, KCl, RbCl, and CsCl, CsCl has the largest electronegativity difference between the two elements involved. Therefore, CsCl is the most ionic compound among the given options.