(c)
Let $e$ and $e'$ are the eccentricities of a hyperbola and its conjugate hyperbola.
Then, $\frac{1}{e^{2}} + \frac{1}{\left( e^{'} \right)^{2}} = 1 \Rightarrow \ \frac{\ 1}{3} + \frac{1}{\left( e^{'} \right)^{2}} = 1 \Rightarrow \ e^{'} = \sqrt{\frac{3}{2}}$