For a hydrogen atom, which of the following transitions emits highest frequency photons? 

answer

According to the energy equation in Bohr's theory

$\mathrm{E}=13.6{\mathrm{Z}}^2\left(\frac{1}{{\mathrm{n}}_1^2}-\frac{1}{{\mathrm{n}}_2^2}\right)=\mathrm{h}\nu$

frequency is maximum, when energy difference is minimum and it is minimum for transition n = 2 to n = 1