Answer: B. observed wavelength of light
Explanation: The relation of Planck's constant relating the Joules of energy absorbed/released by matter is usually used to determine the energy of the photon. The mathematical expression is -
E= h c /

Thus as we can observe that the energy is directly proportional to the speed of light and inversely proportional to the wavelength of the light.
Thus , we can say that Planck's constant relates the Joules of energy absorbed/released by matter to the observed wavelength of light.