In carbonyl compound the (C=O) bond is polarised due to high electronegativity of O atom relative to C atom, as shown below:
If any electron withdrawing group attached with carbon atom of carbonyl group than polarity increases. If any electron releasing group attached with carbon atom of carbonyl group than polarity decreases. Among the given case HCHO doesn't have strong electron releasing group hence it will be most polar. Because in the absence of strong electron releasing group the bond will more polar than other.