Explanation:

Correct option is 4- 66 km/h

Speed of truck for first 4 hours of journey = 90 km/h
Now,
Speed = Distance/Time
Solution
Distance covered in first four hours = 90 x 4
⇒ 360 km
Distance covered in 6 hours = 6 x 50
⇒ 300 km
Total Distance covered = 360 + 300
⇒ 660 km
Total time = 4 + 6
⇒ 10 hours
Average speed = 660/10
⇒ 66 km
:: Average speed of truck is 66 km/h