Correct option is A
In a micrometer, a lock nut (or locking device) is used to fix the spindle in position once the correct measurement is obtained. This allows the user to read the measurement accurately without any movement, ensuring precision.
Other parts have different functions: the sleeve/barrel carries the main scale, the anvil is the fixed measuring face, and the thimble rotates to move the spindle. None of these hold the reading steady.