Firstly calculate num of moles of option A, C, D. No of molecules =No of moles × Avogadro's num. Here, no if moles are max in option B so no of molecules would be max in this. So correct option is B.

Explanation:

18 g of water:
- To find the number of water molecules in 18 g of water, we first need to convert grams to moles.
- The molar mass of water (H2O) is approximately 18 g/mol.
- Therefore, 18 g of water is equal to 1 mole of water.
- One mole of water contains 6.022 x 10^23 molecules (Avogadro's number).
- So, there are 6.022 x 10^23 molecules of water in 18 g of water.

18 moles of water:
- If we have 18 moles of water, we can directly use Avogadro's number to find the number of molecules.
- 1 mole of water contains 6.022 x 10^23 molecules.
- Therefore, 18 moles of water will contain 18 x 6.022 x 10^23 = 1.084 x 10^25 molecules of water.

18 molecules of water:
- This is a very small number of water molecules.
- It is not practically possible to have only 18 molecules of water in a measurable quantity.

1.8 g of water:
- To find the number of water molecules in 1.8 g of water, we again need to convert grams to moles.
- 1.8 g of water is equal to 0.1 moles of water.
- 0.1 moles of water contain 0.1 x 6.022 x 10^23 = 6.022 x 10^22 molecules of water.
Therefore, the option with the maximum number of water molecules is 18 moles of water, which contains 1.084 x 10^25 molecules of water.