Which of the following contain the same number of atoms ? (Given :...

Number of atoms is calculated as the product of the number of moles, atomicity, and Avogadro's number ($N_A$).

For A: Moles of O$_2=\dfrac{2}{32} = \dfrac{1}{16}$ mol. Atomicity of O$_2$ is $2$.
Number of atoms = $\dfrac{1}{16} \times 2 \times N_A = \dfrac{1}{8} N_A$

For B: Moles of SO$_2=\dfrac{4}{64} = \dfrac{1}{16}$ mol. Atomicity of SO$_2$ is $3$.
Number of atoms = $\dfrac{1}{16} \times 3 \times N_A = \dfrac{3}{16} N_A$

For C: Moles of O$_2$ at STP = $\dfrac{1400}{22400} = \dfrac{1}{16}$ mol. Atomicity of O$_2$ is $2$.
Number of atoms = $\dfrac{1}{16} \times 2 \times N_A = \dfrac{1}{8} N_A$

For D: Moles of He at STP = $\dfrac{0.05}{22.4} = \dfrac{1}{448}$ mol. Atomicity of He is $1$.
Number of atoms = $\dfrac{1}{448} \times 1 \times N_A = \dfrac{1}{448} N_A$

For E: Moles of H$_2=0.0625 = \dfrac{1}{16}$ mol. Atomicity of H$_2$ is $2$.
Number of atoms = $\dfrac{1}{16} \times 2 \times N_A = \dfrac{1}{8} N_A$

Comparing the number of atoms, A, C, and E contain the same number of atoms.

Answer: A, C and E only