Lest Count is the minimum length you can measure with that device.
(a) here you have said 20 divisions on sliding scale but you also have to mension that how many divisions on main scale does these 20 divisions cover on main scale to find the least count.
(b) in screw guage, pitch is the distance you can cover in one complete rotation i.e. here you can cover 1mm in one rotaion which you have devided in 100 divisions on circular scale. hence the least count will me 1/100 mm
(c) in an optical instrument, the lengths you can see/measure are of comparable magnitude to the wavelength used. the exact reason for this should wait for you as it is beyond your syllabus