Getting the final velocity of a free fall, we need to use this formula:
vf = g * t
where
g = 9.8 m/s^2
t = time
From the problem given, the final velocity of the stone is
vf = g * t
vf = 9.8 m/s^2 * 8 s
vf = 78.4 m/s
So the correct answer is letter C. 78.4 m/s