The number of water molecules in 1 litre of water is:
A) 18

B) 18×103

C) NA

D) 55.55NA

To find the number of water molecules in 1 litre of water, we need to use the concept of moles and Avogadro's number.

1. **Calculate the number of moles in 1 litre of water:**
- The density of water is approximately 1 g/mL, so 1 litre (1000 mL) of water weighs about 1000 grams.
- The molar mass of water (H₂O) is about 18 grams per mole.
- Thus, the number of moles of water in 1000 grams is \( \frac{1000 \text{ grams}}{18 \text{ grams/mole}} \approx 55.56 \text{ moles} \).

2. **Calculate the number of molecules:**
- Avogadro's number (NA) is \( 6.022 \times 10^{23} \) molecules per mole.
- Therefore, the number of molecules in 55.56 moles is \( 55.56 \times 6.022 \times 10^{23} \approx 3.34 \times 10^{25} \).

So, the closest answer to this is:

**D) 55.55NA**