Answer:
The correct answer is c. the truck driver.
Any object is said to be the motion if its relative distance with respect to a reference point is keep on changing.
For example, a kid travelling through a train might seem to be stationary with respect to the seat. However, he would seem to be moving if we see him in respect to the railway track outside the train.
A reference point is anything (place, person or any object) used to determine if something or someone is in motion.
In the given situation, the distance between truck driver and bulldozer will remain the same throughout the journey as it is being placed on the back of the truck. Hence, the correct answer is truck driver.