(a)
Let the equation of circle is
$x^{2} + y^{2} + 2gx + 2fy + c = 0$
Since, $\left( m,\frac{1}{m} \right)$ lies on this circle
$\therefore\ \ m^{2} + \frac{1}{m^{2}} + 2gm + \frac{2f}{m} + c = 0$
$\Rightarrow m^{4} + 2gm^{3} + cm^{2} + 2fm + 1 = 0$
$\Rightarrow m_{1}m_{2}m_{3}m_{4} = 1$