Identify the instrument having minimum least count out of the following options.

What is the number of significant figures in $0.310\times {10}^3$?

In two different systems of units, acceleration is represented by the same number while velocity is represented by numbers in the ratio $1:3$. The ratios of unit of length and time are, respectively,

Consider the vernier calipers shown below. The instrument has no zero error. If $1 MSD=1 \mathrm{mm}$ and $7 MSD = 8 VSD$, then the length (in $\mathrm{mm}$) of the rod shown in the figure is $a$. Write value of $2a$.

[Given; the $4^{\mathrm{th}} VSD$ coincides with $MSD$]