It is important to remember that power = energy per second.
Potential energy (PE) of water when it reaches a height of 5m = m x g x h
PE = 5 x 10 x 5 = 250J
This is the energy of 5kg of water which is released per second.
Hence, power = 250 J/s or 250 W
We know that all kinetic energy is converted to potential energy when it reaches a height.
Therefore PE = KE = 250 J
KE = ½mv²
250 = 0.5 x 5 x v²
100 = v²
v = 10 m/s
Hence the answer is G.