This is an example of non uniform motion with segments having uniform motion though.
So average speed is given by :
Average speed = Total distance / Total time
= (d1 + d2 + d3 ....) / [(d1 / s1) + (d2/s2) ........]
Let distance from Delhi to Chandigarh = (3/4)d
from Chandigarh to Simla = d [ As mentioned in the question ]
speed from Delhi to Chandigarh = (1/2)s
from Chandigarh to Simla = s
Hence average speed = [(3/4)d + d] / [{(3/4)d / (1/2)s} + (d/s)]
= (7/4)d / [(3/2) + 1] (d/s)
= (7/10)s
But given average speed for entire journey = 49 kmph
Hence
(7/10) s = 49
==> s = 70 which is the same as average speed between Chandigarh to Shimla
Hence D) should be the correct answer.