For two Bohr orbits n1 and n2 which follow the relationships, $\left({\mathrm{n}}_2^2-{\mathrm{n}}_1^2\right)=21$ and $\left({\mathrm{n}}_2-{\mathrm{n}}_1\right)=3$. 

If an electron makes transition from n₂ to n₁ directly then :

longest frequency of Lyman series is emitted

only one spectral line is emitted

third longest wavelength of Balmer series is emitted

longest frequency of Balmer series is emitted