Arrange the following structure of quaternary salt in the decreasing order of stability by solvation.
 

In the aqueous phase the substituted ammonium cations get stabilised not only by electron releasing effect of alkyl group (+I) but also by solvation with water molecule.
The greater the size of the ion, lesser will be the solvation and the less stabilised is the ion. Hence, the order of stability of ions are as follows I>II>III.