Correct Answer - Option 2 : 83.7 km/h
Let 3d be total distance of journey.
As we know, overall average speed of the car = (Total distance covered)/(Total time taken)
= 3d/(d/78 + d/84 + d/90) = 3/{(14 × 15 + 13 × 15 + 14 × 13)/( 6 × 13 × 14 × 15)}
= 3 × 16780/(210 + 195 + 182) = 3 × 16780/587 = 3 × 27.9 = 83.7 km/h