Propanoic acid with $Br_2/P$ yields a dibromo product. Its structure would be:

The reaction of carboxylic acids having $\alpha$-hydrogens with chlorine or bromine in the presence of small amount of red phosphorus is known as the Hell-Volhard-Zelinsky (HVZ) reaction.

1. Specificity: This reaction results in the substitution of $\alpha$-hydrogens (hydrogens attached to the carbon adjacent to the $-COOH$ group) by halogen atoms.
2. Reactant: Propanoic acid is $CH_3-CH_2-COOH$. The $\alpha$-carbon is the $-CH_2-$ group, which contains two $\alpha$-hydrogens.
3. Product: If the reaction yields a 'dibromo' product, both $\alpha$-hydrogens are replaced by bromine atoms.

$CH_3-CH_2-COOH \xrightarrow{Br_2/P} CH_3-C(Br)_2-COOH$

The product is 2,2-Dibromopropanoic acid ($CH_3-CBr_2-COOH$). Option 1 represents $\alpha,\beta$-substitution which is incorrect for HVZ.