Pick out the correct statement from the following:

The wavelength of the emission line spectrum and the absorption line spectrum of a substance are related as:

$A$ beam of ultraviolet light of all wavelengths passes through hydrogen gas at room temperature,in the $x-$ direction. Assume that all photons emitted due to electron transition inside the gas emerge in the $y-$ direction. Let $A$ and $B$ denote the lights emerging from the gas in the $x$ and $y$ directions respectively.

Of the following pairs of species,which one will have the same electronic configuration for both members?

What is the total number of electron pairs in an oxygen molecule $(O_2)$?

Band spectrum is obtained when the source emitting light is in the form of