Answer: B. Person only
Explanation: Frame of reference is a body with respect to which the state of rest or the state of motion of a concerned body is defined.
Here in this question the state of motion of the bodies have been defined by the observer.
An inertial frame of reference may also be called an inertial reference frame, inertial frame, Galilean reference frame, or inertial space.
An Inertial Frame of Reference is one which has no magnitude of acceleration, i.e. according to the Newton's first law of motion the inertial frame has no force acting upon it and it continues to be in the state of rest or moves with a constant velocity with no influence of any external force.
- Here the truck moves with a constant speed which tells only about its magnitude and not the direction whereas velocity also has a direction. So we can't say about its uniformity of the velocity just by knowing that it has a constant speed.