Explanation

(b)
Alkali metals on burning in air give monoxide, peroxide or superoxide.
Li forms monoxide.
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 2Li + \frac{1}{2}O_{2}\ \rightarrow Li_{2}O$
Na form peroxide as well as monoxide.
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 2Na\ + O_{2} \rightarrow Na_{2}O_{2}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 2Na + \frac{1}{2}O_{2} \rightarrow Na_{2}O$
K, Rb, and Cs form superoxide.
$M( = K,\ Rb,\ Cs) + O_{2}\ \rightarrow MO_{2}$