To solve this problem, we need to identify the hydrocarbon (A) that reacts with bromine by substitution to form an alkyl bromide, which is then converted by the Wurtz reaction to a gaseous hydrocarbon containing less than four carbon atoms.$\\$Let's analyze each option:$\\$Option 1: $$CH$_3$-CH$_3 \\$• Ethane reacts with bromine to form ethyl bromide: $$CH$_3$-CH$_2$Br$. \\$• Wurtz reaction of ethyl bromide would give butane: $2$CH$_3$-CH$_2$Br$+ 2$Na$\rightarrow$CH$_3$-CH$_2$-CH$_2$-CH$_3 + 2$NaBr$. \\$• Butane has four carbon atoms, which does not satisfy the condition.$\\$Option 2: $$CH$_2=CH_2 \\$• Ethene does not undergo substitution with bromine; it undergoes addition.$\\$• This option is not suitable.$\\$Option 3: $$CH$\equiv$CH$\\$• Ethyne also undergoes addition reactions, not substitution with bromine.$\\$• This option is not suitable.$\\$Option 4: $$CH$_4 \\$• Methane reacts with bromine by substitution to form methyl bromide: $$CH$_3$Br$. \\$• Wurtz reaction of methyl bromide gives ethane: $2$CH$_3$Br$+ 2$Na$\rightarrow$CH$_3$-CH$_3 + 2$NaBr$. \\$• Ethane is a gaseous hydrocarbon with two carbon atoms, satisfying the condition.$\\$Therefore, the correct option is $$Option 4: $$CH$_4.$