Number of atoms $=\frac{\text { mass }}{\text { molar mass }} \times N_{A}$ x Number of atoms in 1 mole
$\therefore$ Number of atoms in $4.25 \mathrm{g} \mathrm{NH}_{3}$
$$
\begin{array}{l}
=\frac{4.25}{17} \times N_{A} \times 4 \\
=N_{A}
\end{array}
$$
Number of atoms in $8 \mathrm{g}$
$$
\mathrm{O}_{2}=\frac{8}{32} \times \mathrm{N}_{A} \times 2=\frac{N_{\mathrm{A}}}{2}
$$
Number of atoms in $2 \mathrm{g}$
$$
\mathrm{H}_{2}=\frac{2}{2} \times \mathrm{N}_{A} \times 2=2 \mathrm{N}_{\mathrm{A}}
$$
Number of atoms in $4 \mathrm{g}$
$$
\mathrm{He}=\frac{4}{4} \times N_{A} \times 1=N_{A}
$$
Thus, $2 \mathrm{g} \mathrm{H}_{2}$ contains the maximum number of atoms among the given.