If S and V are one main scale and one Vernier scale and n - 1 divisions on the main scale are equivalent to n divisions of the Vernier, then

Least count: 1 MSD- 1 VSD

          $=\mathrm{S}-\mathrm{V}=\mathrm{S}-\left(\frac{\mathrm{n}-1}{\mathrm{n}}\right)\mathrm{S}=\frac{\mathrm{S}}{\mathrm{n}}[\because \mathrm{nV}=(\mathrm{n}-1\left)\mathrm{S}\right]$

It is also called vernier constant.
So choices (1) and (2) are correct.
Choice (a) is wrong and choice (3) is correct, since for all vernier scales similar approach can be used.