Which one will have maximum numbers of water molecules?

The correct option is 18 moles of water.

To determine which option contains the maximum number of water molecules, we can calculate or express the number of water molecules in each of the given choices. Let us evaluate them step-by-step:

1. 18 molecules of water:
This option contains exactly 18 individual molecules of water ($18$ molecules).

2. 1.8 grams of water:
First, we find the number of moles. The molar mass of water ($H_{2}O$) is approximately $18\text{g/mol}$.
Number of moles = $\frac{\text{Mass}}{\text{Molar Mass}}=\frac{1.8\text{g}}{18\text{g/mol}}=0.1\text{mole}$.
Since 1 mole of any substance contains Avogadro's number ($N_A\approx 6.022\times {10}^{23}$) of molecules, the number of molecules here is:
Number of molecules = $0.1\times N_A=6.022\times {10}^{22}$ molecules.

3. 18 grams of water:
Number of moles = $\frac{18\text{g}}{18\text{g/mol}}=1\text{mole}$.
Number of molecules = $1\times N_A=6.022\times {10}^{23}$ molecules.

4. 18 moles of water:
By definition, 18 moles of water contains:
Number of molecules = $18\times N_A=18\times 6.022\times {10}^{23}\approx 1.084\times {10}^{25}$ molecules.

Comparing the quantities:
- 18 molecules ($18$ molecules)
- 1.8 grams of water ($6.022\times {10}^{22}$ molecules)
- 18 grams of water ($6.022\times {10}^{23}$ molecules)
- 18 moles of water ($1.084\times {10}^{25}$ molecules)

Thus, 18 moles of water contains by far the largest number of water molecules.