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

Explanation

(i)Let A be R-H, on bromination , it will give R-Br
`underset"Alkane (A)" (R-H)+ Br_2 overset"hv"to underset"Alkyl bromide"(R-Br)+HBr `
(ii)Now Wurtz reaction of alkyl bromide gives alkane (B)
`R-Br + 2Na + Br-R overset"Heat"to underset"Alkane (B)"(R-R) + 2NaBr`
(iii)Since alkane (B) contains less than four carbon atoms, therefore , R can have only one carbon atom. In other words , R is `CH_3` and hence alkane (A) is `CH_4`
All the reactions involved here are :
`underset"Methane (A)"(CH_4)underset"-HBr"overset(Br_2)to underset"Methyl bromide "(CH_3Br) underset"(Wurtz reaction)"overset"Na, Dry ether"to underset"(Less than 4 carbon atoms )"(CH_3-CH_3)`