Which of the following is the most precise device for measuring length:
$A)$ a vernier callipers with $20$ divisions on the sliding scale
$B)$ a screw gauge of pitch $1 \; mm$ and $100$ divisions on the circular scale
$C)$ an optical instrument that can measure length to within a wavelength of light?

In a vernier callipers,$20$ $VSD$ coincide with $16$ $MSD$ (each division of length $1 \text{ mm}$). The least count of the vernier callipers is: (in $\text{ cm}$)

Consider the diameter of a spherical object being measured with the help of a Vernier callipers. Suppose its $10$ Vernier Scale Divisions $(V.S.D.)$ are equal to its $9$ Main Scale Divisions $(M.S.D.)$. The least division on the $M.S.$ is $0.1 \ cm$ and the zero of $V.S.$ is at $x=0.1 \ cm$ when the jaws of the Vernier callipers are closed. If the main scale reading for the diameter is $M=5 \ cm$ and the number of the coinciding vernier division is $8$,the measured diameter after zero error correction is: (in $cm$)

In a vernier callipers,$10$ divisions of vernier scale coincide with $9$ divisions of main scale,the least count of which is $0.1\,cm$. If in the measurement of inner diameter of a cylinder,the zero of the vernier scale lies between $1.3\,cm$ and $1.4\,cm$ of the main scale and the $2^{nd}$ division of the vernier scale coincides with a main scale division,then the diameter will be .......... $cm$.

Student $A$ and Student $B$ used two screw gauges of equal pitch and $100$ equal circular divisions to measure the radius of a given wire. The actual value of the radius of the wire is $0.322 \, \text{cm}$. The absolute value of the difference between the final circular scale readings observed by the students $A$ and $B$ is .... .
Given pitch $= 0.1 \, \text{cm}$.

The length of a cylinder is measured with the help of vernier callipers whose nine divisions of the main scale are equal to ten divisions of the vernier scale. The smallest division on the main scale is $0.5 \ mm$. It is observed that the zero of the vernier scale has just crossed the $78^{th}$ division of the main scale,and the fourth division of the vernier scale coincides with a main scale division. The length of the cylinder is $..... \ mm$.