Explanation

(a)
Nitrates of alkali metals on heating evolve oxygen gas (e.g., $KNO_{3}$) while nitrates of $p$ and $d$-block elements [e.g., $\left( {NO}_{3} \right)_{2},\ Cu\left( {NO}_{3} \right)_{2}\ and\ {AgNO}_{3}$] gives out nitrogen dioxide on heating
$2{KNO}_{3} \rightarrow 2{KNO}_{2} + O_{2}\$
$\therefore$ Nitrogen dioxide cannot be prepared from ${KNO}_{2}$.