For a first order $Rx_n$ of rate constant K,$\;t_{ 2/3}$ the time of a reactant to drop to $1/3$) is given by:

Question

Note: $ln \;2=0.693$ & $ln\; 3=1.1$

$\begin{array}{1 1}(a)\;0.693/K&(b)\;1.1/k\\(c)\;0.407/K&(d)\;1.793/K\end{array}$