The wavelength of a spectral line for an electronic transition in an atom or molecule is determined by the difference in energy ($E$) between the two energy levels involved in the transition. According to the Planck relation, the energy difference between the two levels is related to the frequency ($f$) of the emitted or absorbed radiation by $E = hf$, where $h$ is Planck's constant. The frequency ($f$) of the radiation is related to its wavelength ($\lambda$) and the speed of light ($c$) by the equation $c = \lambda f$. Combining these equations gives us the relation $\lambda = \frac{hc}{E}$.

Option A, "The number of electrons undergoing the transition," does not directly affect the wavelength of the spectral line. The wavelength is determined by the energy difference between the initial and final states of the transition, not the number of electrons involved.

Option B, "The nuclear charge of the atom," indirectly affects the energy levels of electrons in the atom, particularly for hydrogen-like atoms where the energy levels are indeed dependent on the nuclear charge. However, the primary relationship between wavelength and an atomic property is not directly with nuclear charge but with the energy difference between levels, which can be influenced by nuclear charge among other factors.

Option C, "The difference in the energy of the energy levels involved in the transition," is the correct option. As explained, the wavelength ($\lambda$) of the emitted or absorbed radiation is directly related to the energy difference ($E$) between the two levels involved in the transition by the equation $\lambda = \frac{hc}{E}$, making the relationship inverse as stated in the question.

Option D, "The velocity of the electron undergoing the transition," does not directly determine the wavelength of the spectral line. While the velocity of an electron can influence its kinetic energy, and thus indirectly affect transitions and spectral lines in specific contexts (such as Doppler broadening), the primary relationship is between the wavelength and the energy difference of the levels involved in the transition.

Therefore, the correct answer is Option C: "The difference in the energy of the energy levels involved in the transition." This highlights the inverse relationship between the wavelength of a spectral line and the energy difference of the transition involved.