Answer
$2.4\times10^{-3}\,g/L$
Work Step by Step
Given: $P_{O_{2}}= 100.\,mmHg$ and
$k_{H}=1.5\times10^{-6} mol \,L^{-1} mmHg^{-1}$
According to Henry's law,
$S_{O_{2}}=k_{H} \times P_{O_{2}}$
$=1.5\times10^{-6} mol \,L^{-1} mmHg^{-1}\times100\,mmHg=1.5\times10^{-4}mol/L$
Concentration of $O_{2}$ in g/L=
$1.5\times10^{-4}mol/L\times\frac{16\,g}{1\,mol}=2.4\times10^{-3}\,g/L$