c
We know that power factor is \(\cos \phi\)

\(\cos \phi=\frac{ R }{ Z }\)

\(Z=\sqrt{R^{2}+\left(X_{L}-X_{C}\right)^{2}} \quad \ldots\)

\(\Rightarrow Z =\sqrt{6^{2}+(10-4)^{2}}\)

\(\Rightarrow Z =6 \sqrt{2} \mid \cos \phi=\frac{6}{6 \sqrt{2}}\)

\(\cos \phi=\frac{1}{\sqrt{2}}\)