c
(c) \(t = \frac{A}{a}\sqrt {\frac{2}{g}} \left[ {\sqrt {{H_1}} - \sqrt {{H_2}} } \right]\)
Now, \({T_1} = \frac{A}{a}\sqrt {\frac{2}{g}} \left[ {\sqrt H - \sqrt {\frac{H}{\eta }} } \right]\)
and \({T_2} = \frac{A}{a}\sqrt {\frac{2}{g}} \left[ {\sqrt {\frac{H}{\eta }} - \sqrt 0 } \right]\)
According to problem \({T_1} = {T_2}\)
\(\sqrt H - \sqrt {\frac{H}{\eta }} = \sqrt {\frac{H}{\eta }} - 0\)==> \(\sqrt H = 2\sqrt {\frac{H}{\eta }} \Rightarrow \eta = 4\)