Observed reading = M.S.R. + ________ .
A) $M.S.D. \times L.C.$
B) $L.C.$
C) $V.S.D. \times L.C.$
D) $None$

Hint: To find the correct observed reading we need to find the least count first, Least count is the smallest value that can be measured by the measuring instrument.

Complete step by step solution:
$LC = MSD - VSD$ is called lowest number or vernier constant. This is the shortest length that can be accurately measured with Vernier calipers. For the given vernier calipers.
The formula for writing the observed reading is, observed reading=$M.S.R.+V.S.D. \times L.C.$

Additional Information:
We will first determine the Vernier constant (VC), which is the lowest (LC) of the vernier caliper, and it writes the steps as equations, LC = 1 MSD - 1 VSD. Now, near displacement move the jaw, with fixed jaws and find zero error. Do this three times and enter the value. If there are no errors, write 'zero error zero'.
Open the jaw of the vernier caliper and place the sphere or cylinder between the two jaws and adjust the movable jaw, allowing it to hold the body gently without any pressure. This done, tighten the screw attached to the Vernier scale.
Look at the position of the zero mark of the Vernier scale on the main scale. Record the reading on the main scale just before the zero mark of the Vernier scale. This reading (N) is called the main scale reading (MSR). .
Note the number (n) of the vernier scale division that coincides with the division of the main scale. We should repeat steps 5 and 6 after measuring the diameter in the vertical direction by the body to 90, repeat steps 4 to 7 for three different positions and record the observations. Now find the total readings using the equation, $TR = MSR + VSR = N + (N \times LC)$ and apply the zero correction. Find the mean of different values ​​of diameter and show the result with the appropriate unit.

Note: Note that Vernier calipers can be used to measure:
a) External dimensions such as the diameter of a sphere or the edge of a cube
b) Internal dimensions such as the internal diameter of a hollow cylinder and
c) The depth of a hollow Cylinder.