c
\(\left(a^{2}+b^{2}+2 a b \cos \theta\right)=4\left(a^{2}+b^{2}-2 a b \cos \theta\right)\)

put \(a\) = \(b\) we get

\(2 a^{2}+2 a^{2} \cos \theta=8 a^{2}-8 a^{2} \cos \theta\)

\(\cos \theta=\frac{3}{5}\)