(c) From Biot-Savart law the magnetic field at the centre is directly proportional to the length of current carrying segment.
$\therefore\frac{B_{1}}{B_{2}} = \frac{length\ of\ ABC}{length\ of\ ADC}$
$\ \ \ \ \ \ \ \ \ \ = \frac{angle\ subtended\ by\ ABC}{angle\ subtended\ by\ ADC}$
$\ \ \ \ \ \ \ \ \ \ = \frac{(360{^\circ} - 60{^\circ})}{60{^\circ}} = \frac{300}{60} = \frac{5}{1}$