c
\(\begin{array}{l}
{y_1} = \frac{1}{2}g{n^2},\,{y_2} = \frac{1}{2}{\left( {n - N} \right)^2}\\
\therefore \,\,{y_1} - {y_2} = \frac{1}{2}g\left[ {{n^2} - {{\left( {n - N} \right)}^2}} \right]\\
\Rightarrow \,1 = \frac{g}{2}\left( {2n - N} \right)N\\
\left[ {\,{y_1} - {y_2} = 1} \right]\\
\Rightarrow \,n = \frac{1}{{gN}} + \frac{N}{2}
\end{array}\)