b
\({v^2} - {u^2} = 2as \Rightarrow {v^2} - {0^2} = 2 \times 9.8 \times 19.6\)

(Initial velocity \(= 0\) as water falls from rest)

\(\begin{array}{l}
 \Rightarrow \,v = \sqrt {2 \times 9.8 \times 2 \times 9.8} \\
\,\,\,\,\,\,\,\,\,\, = 2 \times 9.8 = 19.6\,m/s
\end{array}\)