A precipitate formed in a reaction or solution is when we have a solid salt that is not soluble in the reaction anymore, in our case we are dealing only with the compound formed with Sodium and another anion, so the one that is not soluble in water will be the one that will form a precipitate, therefore a solid salt not dissolvable. Out of the three ionic compounds: Na2CO3, Na2CrO4, NaI2, Sodium chromate (Na2CrO4) will present as the less soluble in water, since chromates in general are not very soluble in water, which is not the case for carbonates and iodides, they will readily dissolve in water, not forming any precipitates