Is it true that object P is retarding and object Q is accelerating in the given figure?

The given statement is true.
In a velocity-time graph, a straight line represents acceleration, so both graphs show acceleration, but the initial and final velocity of the objects decide whether they are accelerating or decelerating, so, for object P, the initial velocity is more than the final velocity and therefore it is decelerating and in second graph object Q has final velocity more than initial velocity and therefore it is accelerating.
As we know, if the slope is positive, then its acceleration, and if the slope is negative, then it will be deceleration.
Mathematically
$\mathit{Acceleration }= \frac{\mathit{change\; in\; velocity}}{\mathit{ change\; in\; time}}$
For P,
$\mathit{Slope}= \frac{\mathit{final\; velocity}-\mathit{intial\; velocity}}{\mathit{Time\; interval}}$
           $=\frac{0-15}{5}$
           $=-3$.
Here slope is negative so it is deceleration
For Q,
$\mathit{Slope}= \frac{\mathit{final\; velocity}-\mathit{intial\; velocity}}{\mathit{Time\; interval}}$
           $=\frac{5-0}{5}$
           $=1.$
Here slope is positive so it is acceleration.