Among the following, the smallest unit of time is            

1 milli second$={10}^{-3}\text{s}$  

1micro second$={10}^{-6}\text{s}$

1 nano second $={10}^{-9}\text{s}$  

1shake$={10}^{-8}\text{s}$  

From the above values we get the smallest unit of time is nano second