Which of the following characteristics remains unchanged when light...

When light undergoes refraction, it travels from one medium to another. The frequency of light depends only on the source of the light and not on the medium through which it travels. Therefore, frequency remains unchanged.

The velocity of light changes according to the refractive index of the medium, given by $v = \dfrac{c}{n}$.

Since $v = f \lambda$ and frequency $f$ is constant, the wavelength $\lambda$ also changes when velocity changes.

The amplitude of light generally decreases during refraction because a portion of the incident light is reflected at the boundary of the two media, reducing the energy and intensity of the refracted wave.