Total distance covered by the bus =39+25

   =64 km/hr

Total time taken to travel= $\left(\frac{45}{60}\right)\mathrm{hr}+\left(\frac{35}{60}\right)\mathrm{hr}=\frac{4}{3}\mathrm{hr}$

Average speed =$\frac{\mathrm{total} \mathrm{distance}}{\mathrm{total} \mathrm{time}}$

Average speed of the bus =$\frac{64}{{\displaystyle \left(\frac{4}{3}\right)}}$= 48 km/hr

Hence, Option D is correct.