Answer
See the figures below.
Work Step by Step
In this problem, the author asks us for 3 sketches.
a) The motion diagram.
b) The force diagram.
c) The free body diagram.
The tricky thing here is what makes the box accelerate since it has the same acceleration as the truck.
When the truck speeds up, the box will be on the verge of sliding to the back of the truck's bed but the force that prevents it from sliding is the static friction force between the box and the bed of the truck.
And since the box is on the verge of sliding to the left, the static friction force had to be to the right (as we know the friction force is always opposing the direction of the motion of the objects).
And hence, the static friction force is the cause of the acceleration of the box.
See the figures below.
