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

Gaseous bromine reacts with alkanes via a free radical substitution mechanism. Among the options:${\text{C}}_2{\text{H}}_4$ (ethene), ${\text{C}}_3{\text{H}}_6$ (propene), and ${\text{C}}_2{\text{H}}_2$ (ethyne) are unsaturated and typically undergo addition reactions with liquid bromine.${\text{C}}_4{\text{H}}_{10}$ (butane) is an alkane. In the presence of sunlight or heat, gaseous bromine reacts with alkanes to form alkyl bromides. Substitution in alkanes happens more readily in the gaseous phase compared to addition in unsaturated systems under these specific conditions.