(`A`) Vernier callipers with `20` divisions on sliding scale, coinciding with `19` main scale divisions
(`B`) A screw gauge of pitch `1 mm` and `100` divisons on the circular scale
(`C`) An optical instrument that can measure length to within a wavelength of light Out of `A`, `B` and `C` the most precise devide for measuring length is

Explanation

A device with minimum count is the most suitable to measure length. (a) Least count of vernier callipers = 1 standard division (SD) – 1 vernier division (VD)
=`1-9/10=1/10=0.01 cm `
(b) Least count of screw gauge =`("Pitch")/("number of divisions")`
`=1/1000=0.001 cm `
(c) Least count of an optical device =wavelength of lifhg ~ `10^(-5) cm`
=0.00001 cm
Hence, it can be inferred that an optical instrument is the most suitable device to measure length.