In Wacker process, alkene is oxidised into aldehyde. CH2=CH2+
1
2
O2
PbCl2.CuCl2
────────▶
H2O
CH3
CHO
(B)
Since only alkenes produce aldehydes, on ozonolysis, hence 'A' must be an alkene. Now to find the structure of alkene we should add two molecules of aldehyde and replace O by double bond.