Mass spectrum is plot in which y-axis is taken as intensity and x-axis is taken as m/z also known as mass to charge ratio in order to analyse a compound.
The mass to charge ratio depends upon the molecular mass of a compound or the corresponding cation fragments while, the intensity mainly depends upon the stability and concentration (natural abundance) of atoms / ions respectively.
In said problem we can distinguish 2,2-dimethylpropane from Pentane and Isopentane by different ways using mass spectrum. As shown in attached picture all of the given compounds give a same m/z value of 57. As the carbocation formed by 2,2-dimethylpropane is tertiary so its intensity will be greater because this cation is more stable. Therefore, the peak m/z = 57 for Pentane (primary carbocation) and Isopentane (secondary carbocation) will be having less intensity than 2,2-dimethylpentane.
