The basic strength of amines depends on the availability of the lone pair of electrons on the nitrogen atom for donation. Electron-donating groups (EDG) increase the electron density on nitrogen, making the amine more basic. Conversely, electron-withdrawing groups (EWG) decrease the electron density, making the amine less basic. In the standard NEET 2017 question this refers to, the structures are typically (I) Aniline, (II) p-Nitroaniline (which has a strongly electron-withdrawing $-NO_2$ group), and (III) p-Toluidine (which has an electron-donating $-CH_3$ group). Therefore, the p-nitroaniline is the least basic, and p-toluidine is the most basic. The correct increasing order is II < I < III.