Achromatic aberration in a lens occurs because (a) different wavelengths of light have different indices of refraction, (b) the shape of the lens is not correctly curved so that different light paths don't necessarily converge at the same point, (c) light focused along two different perpendicular planes does not converge at the same point, (d) the lens does not follow a proper moral standard.

option (a)

Explanation:

Chromatic means colour

it means it is the defect due to the colour of light.

As we know that there are seven colours of light which have different wavelength and different refractive index. As they have different refractive index, so they focus at different points on the principal axis of the lens.

As the different colours of light focus at different points so a blurred and a coloured image is seen and the defect is called chromatic aberration.