How many of the following will turn moist red litmus blue and finally white ?

$L{i}_{2}O,K{O}_3,Rb{O}_2,C{S}_2{O}_2,BeO,MgO,Ba{O}_2,SrO$

The standard oxides Li₂O, MgO and SrO are water-soluble and hydrolyse to form hydroxides and a basic solution. BeO is insoluble in water.

RbO₂ is superoxide which in aqueous solution undergoes disproportionation to O₂ and OH- so will give a blue litmus test.

The peroxides Cs₂O₂ and BaO₂ react with water to give hydrogen peroxide and the hydroxides, so again a basic solution. The hydrogen peroxide produced will bleach the litmus paper.

KO₃ - Potassium Ozonide, highly unstable, decomposes in water to oxygen and KOH.

The oxides which will NOT decolourise litmus are Li₂O, MgO, BeO and SrO. The peroxides (Cs₂O₂ and BaO₂), the superoxide (RbO₂₎ and the ozonide (KO3​) will decolourise litmus.