
(c) The total time from $A$ to $C$
$t_{AC} = t_{AB} + t_{BC}$
$= \left( \frac{T}{4} \right) + t_{BC}$
Where $T =$time period of oscillation of spring mass system
$t_{BC}$ can be obtained from, $BC = AB\sin{(2\pi/T)}t_{BC}$
Putting $\frac{BC}{AB} = \frac{1}{2}$ we obtain $t_{BC} = \frac{T}{12}$
$\Rightarrow \ \ \ t_{AC} = \frac{T}{4} + \frac{T}{12} = \frac{2\pi}{3}\sqrt{\frac{m}{k}}$