The lowest freezing point is given by the solute which has highest depression in freezing point. Depression in freezing point is a colligative property which depends upon the number of ions. Depression in freezing point $ \propto $ number of molecules or ions. In given alternatives only $ CaC{{l}_{2}} $ gives maximum number of ion (3 ions). So, it has maximum depression in freezing point. Therefore, $ 0.1\text{ }m\text{ }CaC{{l}_{2}} $ solution has lowest freezing point.