What is the correct stability order of $\mathrm{KO}_2, \mathrm{RbO}_2, \mathrm{CsO}_2$ ?

Assertion (A) $\mathrm{MgO}, \mathrm{CaO}, \mathrm{SrO}$ and BaO are insoluble in water.

Reason ( R ) In aqueous medium the basic strength of $\mathrm{MgO}, \mathrm{CaO}, \mathrm{SrO}$ and BaO increases with increase in the atomic number of metal.

The correct option among the following is

Identify the element for which +1 oxidation state is more stable than +3 oxidation state.

Observe the oxides $\mathrm{CO}, \mathrm{B}_2 \mathrm{O}_3, \mathrm{SiO}_2, \mathrm{CO}_2, \mathrm{Al}_2 \mathrm{O}_3$. $\mathrm{PbO}_2, \mathrm{Tl}_2 \mathrm{O}_3$. The number of acidic oxides in the list is