The equation of motion of a particle started at t=0, is given by x= 5sin $\left(20\mathrm{t}+\frac{\mathrm{\pi }}{3}\right)\mathrm{cm}.$ The least time after which acceleration becomes zero is 

Given,

$x=5\sin \left(20\mathrm{t}+\frac{\mathrm{\pi }}{3}\right)$

we know that,

$v=\frac{dx}{dt}=100\cos \left(20t+\frac{\mathrm{\pi }}{3}\right)$

$a=\frac{dv}{dt}=-2000\sin \left(20t+\frac{\mathrm{\pi }}{3}\right)$

acceleration is zero for the least time when,

$20t+\frac{\mathrm{\pi }}{3}=\mathrm{\pi }$

$t=\frac{\mathrm{\pi }}{30}s$

Hence the correct answer is $\frac{\mathrm{\pi }}{30}s.$