Answer
Choice A.
Work Step by Step
According to the given expression, the spacing between adjacent energy levels is proportional to $\frac{1}{M}$.
Therefore, if the mass M increases, the energy spacing $\Delta E$ decreases.
We know that the energy spacing is related to the wavelength of the emitted photon:$\Delta E = \frac{hc}{\lambda}$.
If $\Delta E$ decreases, then the wavelength increases, which is what is observed.
This analysis supports choice A.
