Polarity is determined by the difference in electronegativity between atoms in a molecule.

Butyric acid (CH3CH2CH2COOH) has a carboxylic acid group (-COOH), which is highly polar due to the presence of an electronegative oxygen atom. The oxygen atom attracts electrons towards itself, creating a partial negative charge (δ-) on the oxygen and a partial positive charge (δ+) on the carbon atom.

Phenyl acetate (C6H5COOCH3) has an ester group (-COOCH3), which is also polar but less polar than a carboxylic acid group. The ester group has a similar structure to the carboxylic acid group, but it lacks the hydrogen atom attached to the oxygen. This difference in structure makes the ester group less polar.

Therefore, butyric acid, with its carboxylic acid group, is more polar than phenyl acetate, which has an ester group.