(b)
This is Wurtz reaction. In this reaction two molecules of alkyl halide react with each other to form alkane having double the number of carbon atoms.
$2CH_{3}CH_{2}Cl + 2Na\overset{\ \ \ Dry\ ether\ \ }{\rightarrow}CH_{3}CH_{2}CH_{2}CH_{3} + 2NaCl$
$\ \ \ \ \ \ \ (X)\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \$ butane
ethyl chloride