$\begin{array}{1 1}(a)\;3.010\times 10^{22}&(b)\;6.02\times 10^{23}\\(c)\;2.010\times 10^{23}&(d)\;4.01\times 10^{22}\end{array}$

Gram-molecular mass of water=18g

Number of moles in 0.9g of water =$\large\frac{0.9}{18}$

$\Rightarrow 0.05$

Number of water molecules in one mole of water=$6.02\times 10^{23}$

Number of molecules of water in 0.05 moles

$\Rightarrow 0.05\times 6.02\times 10^{23}$

$\Rightarrow 3.010\times 10^{22}$

As one molecule of water contains one oxygen atom ,so number of oxygen atoms in $3.010\times 10^{22}$ molecules of water =$3.010\times 10^{22}$

Hence (a) is the correct answer.

Thanks for the answer

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