The correct answer is $\boxed{\text{C}}$.
To maximize the range of a projectile, it must be thrown at an angle of 45 degrees to the horizontal. This is because the horizontal component of the velocity of the projectile will remain constant throughout its flight, while the vertical component will decrease due to gravity. The maximum range is achieved when the vertical component of the velocity of the projectile is zero at the point of impact. This occurs when the projectile is thrown at an angle of 45 degrees to the horizontal.
In the case of the stone being thrown up a slope of inclination 60° to the horizontal, the angle at which the stone must be thrown to land as far as possible from the point of projection is also 45 degrees to the horizontal. This is because the slope does not affect the horizontal component of the velocity of the projectile. The only effect of the slope is to increase the vertical component of the velocity of the projectile, which will cause the stone to land at a higher point on the slope. However, this will not affect the range of the projectile.
The other options are incorrect because they are not the angles at which the stone must be thrown to land as far as possible from the point of projection.