Answer
The angle that produces the maximum range is 45$^{\circ}$.
Work Step by Step
At 45$^{\circ}$, the x and y components of the projectiles velocity are equal. Any angle above will cause the y-component to be greater and the projectile will fall short. Likewise, any angle below 45$^{\circ}$ will cause the x-component to be greater and the projectile will fall short.
