Which of the following has minimum number of molecules?

The correct option is B

16 g of ${\mathrm{O}}_2$

As we know

Number of molecules =$\frac{givenmass}{molarmass}\times N_A$

Explanation of part (a) :

Option (a) For ${\mathrm{H}}_2\mathrm{O}$

Number of molecules =$\frac{18g}{18g/mol}\times 6.022\times {10}^{23}$

=$6.022\times {10}^{23}$

Explanation of part (b):

Option (b) For ${\mathrm{O}}_2$:

Number of molecules=$\frac{16g}{32g/mol}\times 6.022\times {10}^{23}$

=$3.011\times {10}^{23}$mol

Explanation of part (c):

Option (c) For ${\mathrm{CH}}_4$:

Number of molecules=$\frac{32g}{16g/mol}\times 6.022\times {10}^{23}$

=$12.044\times {10}^{23}$

Explanation of part (d):

Option (d) for ${\mathrm{CO}}_2$:

Number of molecules=$\frac{88g}{44g/mol}\times 6.022\times {10}^{23}$

=$12.044\times {10}^{23}$

Hence, the minimum number of molecules in oxygen is $3.011\times {10}^{23}$

So, the correct option is (b)-16 g of ${\mathrm{O}}_2$