To determine the largest number of molecules in the given options, we need to compare the moles of each substance since the number of molecules depends on the moles of the substance. The formula to calculate the number of moles is given as:

$ \text{Number of moles} = \frac{\text{Mass of substance}}{\text{Molar mass of substance}} $

Let's calculate the moles for each substance:

Option A: 32 g of water (H₂O)

The molar mass of water = (2 × 1) + 16 = 18 g/mol

Moles of water = $\frac{32}{18}$

Moles of water ≈ 1.78 mol

Option B: 28 g of carbon monoxide (CO)

The molar mass of CO = 12 + 16 = 28 g/mol

Moles of CO = $\frac{28}{28}$

Moles of CO = 1 mol

Option C: 46 g of ethyl alcohol (C₂H₅OH)

The molar mass of ethyl alcohol = (2 × 12) + (6 × 1) + 16 + 1 = 46 g/mol

Moles of ethyl alcohol = $\frac{46}{46}$

Moles of ethyl alcohol = 1 mol

Option D: 54 g of nitrogen pentoxide (N₂O₅)

The molar mass of N₂O₅ = (2 × 14) + (5 × 16) = 108 g/mol

Moles of N₂O₅ = $\frac{54}{108}$

Moles of N₂O₅ ≈ 0.5 mol

Given that 1 mole of any substance contains $6.022 \times 10^{23}$ molecules (Avogadro's number), the substance with the highest number of moles will have the most molecules.

Therefore, Option A (32 g of water) has the largest number of molecules among the given options because it has the highest number of moles, approximately 1.78 mol.