Answer: The correct answer is 0.25 mph/s.
Step-by-step explanation:
The expression for the acceleration is as follows:

Here, a is the acceleration, v is the final velocity and u is the initial velocity.
It is given in the problem that Derek is riding his bike 10 mph. He takes 20 seconds to reach 15 mph.
Calculate the acceleration.
Put t= 20 s, v= 15 mph and u= 10 mph in the above expression.


Therefore, his acceleration is 0.25 mph/s.