Answer
Situation (a) has the greatest final speed.
Work Step by Step
Based off of the figure, the only difference between the situations is the height of the landing plane. Situation (a) has the projectile landing at the lowest position, meaning that it spends the most time falling in the air. If a projectile has more time to fall, that means that gravity will have the most time to accelerate its speed. So, situation (a) has the greatest final speed because the projectile has the longest amount of time to fall.
