For a wave propagating in a medium, identify the property that is independent of the others.

As $v=v \lambda$, the velocity $=$ frequency $\times$ wavelength. When light is travelling in a medium, $v$ is assumed to be the same and the velocity of light in the medium is less. $\lambda$ gets adjusted so that $\frac{c^{\prime}}{\lambda^{\prime}}=v$. Here $v$ is independent. It is frequency of light i.e. energy that is a constant. Wavelength and velocity change. $\therefore c / \lambda=$ constant.