Which of the following compounds is the least soluble in water?

The number of the following factors which affect the percent covalent character of the ionic bond is $............$.
$(a)$ Polarising power of cation
$(b)$ Extent of distortion of anion
$(c)$ Polarisability of the anion
$(d)$ Polarising power of anion

Which compound has the lowest melting point?

According to $Fajan's \, rule$,covalent character is favoured by:

How would you explain the following observations?
$(i)$ $BeO$ is almost insoluble but $BeSO_4$ is soluble in water.
$(ii)$ $BaO$ is soluble but $BaSO_4$ is insoluble in water.
$(iii)$ $LiI$ is more soluble than $KI$ in ethanol.

Which compound has the highest covalent character?