Explanation

(a)
$- OCH_{3}$ is strongest electron releasing group (+M effect) which opposes most the dispersion of lone pair of electron of nitrogen into the ring. Thus,$\ - OCH_{3}$ being at para position imparts hifhest basicity. $NO_{2}$ being at meta position stabilises the electron pair of nitrogen only by –I effect. While $NO_{2}$being present at para position due to –M effect and –I effect stabilizes the lone pair of electron of nitrogen, most and impart least basicity.
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