a)

Wrap the thread so that the coils formed are close to each other on a uniform metre scale. Using a metre scale, determine the length(l) of the thread. If the thread's diameter is d, and the number of turns obtained is n. The thread's diameter is determined by the relationship.
\text{ Diameter of the thread} = \frac{\text{ length(l) of the thread }}{\text{ number of turns }}
b)
Theoretically:
The accuracy of an instrument is inversely proportional to the instrument's least count.
\text{ Least count screw gauge }=\frac{\text{ pitch of screw gauge }}{\text{ number divisions of the circular scale }}
Increasing the number of divisions on the circular divisions reduces the value of least count, and thus improves accuracy.
Practically:
However, the number of divisions can only be raised accuracy to a certain extent.

c)
A set of 100 measures is more accurate than 5, since random errors in the former are much fewer than the latter.