a
We know that, for series \(R-C\) circuit

\(V^{2}=V_{C}^{2}+V_{R}^{2}\)

\(100=64+V_{R}^{2}\)

\(\Rightarrow V_{R}^{2}=36 \Rightarrow V_{R}=\sqrt{36}=6 V\)

Also, tan \(\phi=\frac{V_{C}}{V_{R}} \Rightarrow \tan \phi=\frac{8}{6}\)

\(\tan \phi=\frac{4}{3}\)

\(\therefore \phi=\tan ^{-1}\left(\frac{4}{3}\right)\)