A plane glass slab is kept over coloured letters, the letter which appears least raised is

When a plane glass slab is placed over coloured letters, the letters appear raised due to the phenomenon of refraction of light.

The apparent shift or raised appearance of the letters depends on the refractive index of the glass for different colours of light.

The refractive index (μ) of glass is not the same for all colours — it varies with wavelength due to dispersion of light.

Violet light has the shortest wavelength and is refracted the most, hence it has the highest refractive index in glass.

On the other hand, red light has the longest wavelength and is refracted the least, having the lowest refractive index.

The apparent depth (and hence the raised appearance) of the image is inversely proportional to the refractive index. Therefore, the greater the refractive index, the less raised the image appears.

Hence, among all colours, the violet letter appears the least raised because it has the maximum refractive index.

Therefore, the correct answer is Option B: violet.