The molecular formula C_3H_6O which follows general molecular formula C_nH_(2n)O represents either a saturated aldhyde or a saturated ketone.A compound containing ketomethyl group (CH_3CO-) linked with H atom or hydrocarbon group responds to iodoform test and a compound containing -CHO group responds to Tollen's reagent.A reduces this reagent but B does not.So A contains -CHO group.Again B undergoes iodoform test but A does not.So it contains CH_3CO- group but not -CHO
So" "A->CH_3CH_2CHO->"propanal"
and B->CH_3COCH_3->"propanone"