(a) the diameter of a thread cannot be found by a meter road as such . it may be found by winding a known number of turns over a pencil , the turns touching each other closely . then the length occupied by a single turn on pentil is equal to the diameter of thread the diameter of thread is also equal to the total thickness divided by total number of folds.
(b)the least count of a screw . whose pitch is 1.0 mm and which has 200 division on its circular scale is given by
least count = 1/200=0.005mm
yes it is possible to increase the accuracy of screw gauge by increasing the number of division.
(c) when the diameter of a thin brass road is measured with a vernier calliper
one may got different readings due to following reasons
1 the diameter of road may be different at different places
2 the road may be held between the jaws of vernier calliper bu applying uneven pressure.
such an error is known as random error . therefore if 5 observations are taken then mean of 5 observation may not be very close to the most accurate observation . however if 100 observation are taken then their mean will be very close to most accurate value.