The pair of elements that forms a bond with the least ionic character is a. na and cl b. k and cl c. o and cl d. mg and cl

As we know the electronegativity difference between two atoms in ionic compounds is greater than 1.7. So, greater the difference in electronegativity above 1.7 greater will be the ionic character and vice versa.

For Na and Cl;

                                    E.N of Chlorine   =  3.16
 
                                    E.N of Sodium    =  0.93
                                                                 
                                    Difference               2.23

For K and Cl;

                                    E.N of Chlorine       =  3.16
 
                                    E.N of Potassium    =  0.82
                                                                     
                                    Difference                    2.34

For O and Cl;

                                    E.N of Oxygen     =  3.44
 
                                    E.N of Chlorine    =  3.16
                                                                  
                                    Difference                0.28

For Mg and Cl;

                                    E.N of Chlorine         =  3.16
 
                                    E.N of Magnesium    =  0.93
                                                                       
                                    Difference                     1.31

Result:
           As calculated above,the least electronegativity difference is found between Oxygen and Chlorine which is less than 1.7. Hence, it is not ionic compound or it has least Ionic character.

Spontaneity tell us the direction of the reaction, but not how fast it goes. gibbs free energy brings together enthalpy, entropy and temperature in one easy formula. g = h - ts. and, finally the sign of delta g is always negative for a spontaneous reaction.

Answer is to avoid experimental error