now i think the answer is right .

let the density of the liquid  flowing be  p.

A be the area of cross section

v be the velocity of the liquid flowing thrugh a smal length dx which is practically a constant fot streamline flow.

force on the given element = = pAv * v  (m=pAdx)

power applied on the given element = f.v = pA

 the mass of flow must double  with same time so the velocity must be doubled as the mass flow rate is directly prop to the velocity and others are constant.

now power   so the power has to 8 times.

rate if you find useful.