A beam of white light is allowed to pass through convex lens...

When a beam of white light passes through a convex lens, it undergoes refraction. The refractive index of the lens material varies for different wavelengths of light, being maximum for violet and minimum for red.

Since the focal length of a lens depends on the refractive index according to the lens maker's formula $\dfrac{1}{f} = (\mu -1) \left(\dfrac{1}{R_1} - \dfrac{1}{R_2}\right),$different colours have different focal lengths. Violet light converges closer to the lens, while red light converges farther away.

This inability of a lens to focus all colours of white light at a single point on the principal axis is known as chromatic aberration.

Answer: chromatic aberration