Hydrocarbon ($A$) reacts with bromine by substitution to form an alkyl bromide which by Wurtz reaction is converted to gaseous hydrocarbon containing less than four carbon atoms. ($A$) is:

1. A substitution reaction with ${\text{Br}}_2$ suggests $A$ is an alkane.2. If $A={\text{CH}}_4$, it forms ${\text{CH}}_3\text{Br}$.3. Wurtz reaction: $2{\text{CH}}_3\text{Br}+2\text{Na}\to {\text{CH}}_3-{\text{CH}}_3+2\text{NaBr}$.4. Ethane (${\text{C}}_2{\text{H}}_6$) is a gaseous hydrocarbon with 2 carbon atoms (less than 4).5. If $A={\text{C}}_2{\text{H}}_6$, Wurtz reaction would produce Butane (${\text{C}}_4{\text{H}}_{10}$), which has 4 carbons.Thus, $A$ must be Methane.