The number of water molecules is maximum in:

Calculating number of molecules:$18\text{g}$ water $=\frac{18}{18}=1$ mole $=6.022\times 10^{23}$ molecules.$18$ moles water $=18\times 6.022\times 10^{23}$ molecules.$18$ molecules $=18$ molecules.$1.8\text{g}$ water $=\frac{1.8}{18}=0.1$ mole $=0.6022\times 10^{23}$ molecules.Clearly, $18$ moles of water contains the maximum number of molecules.