A pendulum with time period of 1s is losing energy due to damping. At certain time its energy is $45\;J$. If after completing 15 oscillation , its energy has become $15\;J$ its damping constant (in $s^{-1})$ is

Question

$\begin{array}{1 1} \frac{1}{30} \ln 3\\\frac{1}{15}\ln 3 \\ 2 \\ \frac{1}{2} \end{array}$