The energy levels of an hydrogen atom are shown below. The...

Wavelength of photons released during de-excitation is given by,

$\dfrac{1}{\lambda} = R\left[\dfrac{1}{{\left({n}_{1}\right)}^{2}} - \dfrac{1}{{\left({n}_{2}\right)}^{2}}\right]$.

Clearly, shortest wavelength will correspond to highest energy difference in de-excitation, which in the given figure will correspond to transition from $n = 3$ to $n = 1 .$