Let us calculate the number of molecules in each given option using the Avogadro constant ($N_A = 6.022 \times 10^{23}$):

Option A: It contains exactly $18$ molecules of water.

Option B: Number of moles in $1.8\text{ g}$ of water $= \frac{\text{Mass}}{\text{Molar mass}} = \frac{1.8}{18} = 0.1\text{ moles}$. Number of molecules $= 0.1 \times N_A$.

Option C: Number of moles in $18\text{ g}$ of water $= \frac{18}{18} = 1\text{ mole}$. Number of molecules $= 1 \times N_A$.

Option D: Number of molecules in $18\text{ moles}$ of water $= 18 \times N_A$.

Comparing all the values, $18 \times N_A$ is the largest. Therefore, $18\text{ moles of water}$ contains the maximum number of water molecules.