The compound that will react most readily with gaseous bromine has the formula:

Reaction with gaseous bromine is an electrophilic addition reaction. Alkenes (like $\text{C}_3\text{H}_6$ and $\text{C}_2\text{H}_4$) are generally more reactive than alkynes ($\text{C}_2\text{H}_2$) towards electrophilic addition because the $\pi$-electrons in alkynes are more tightly held by the more electronegative sp-hybridised carbon atoms. Alkanes ($\text{C}_4\text{H}_{10}$) do not undergo addition reactions and only react via free-radical substitution under drastic conditions (e.g., UV light or heat). Between the two alkenes, propene ($\text{C}_3\text{H}_6$) is more reactive than ethene ($\text{C}_2\text{H}_4$). This is because the electron-donating methyl group in propene exerts a $+I$ (inductive) effect and hyperconjugation, which increases the electron density on the carbon-carbon double bond, making it more susceptible to electrophilic attack by $\text{Br}_2$.