a
As we know, frequency in $SHM$
$\mathrm{f}=\frac{1}{2 \pi} \sqrt{\frac{\mathrm{k}}{\mathrm{m}}}=10^{12}$ where $\mathrm{m}=\mathrm{mass}$ of one atom
Mass of one atom of silver, $=\frac{108}{\left(6.02 \times 10^{23}\right)} \times 10^{-3} \mathrm{kg}$
$\frac{1}{2 \pi} \sqrt{\frac{\mathrm{k}}{108 \times 10^{-3}} \times 6.02 \times 10^{23}}=10^{12}$
Solving we get, spring constant, $\mathrm{K}=7.1 \mathrm{N} / \mathrm{m}$