The molecular formula ${C}_{3} {H}_{6} O$ which follows general molecular formula ${C}_{n} {H}_{2 n} O$ represents either a saturated aldhyde or a saturated ketone.A compound containing ketomethyl group ($C {H}_{3} C O -$) linked with H atom or hydrocarbon group responds to iodoform test and a compound containing $- C H O$ group responds to Tollen's reagent.A reduces this reagent but B does not.So A contains $- C H O$ group.Again B undergoes iodoform test but A does not.So it contains $C {H}_{3} C O -$ group but not $- C H O$
So$\text{ "A->CH_3CH_2CHO->"propanal}$
and $B \to C {H}_{3} C O C {H}_{3} \to \text{propanone}$