Which of the following is least reactive in a nucleophilic substitution reaction ?

$(a)\;(CH_3)_3C -Cl \\ (b)\;CH_2 =CHCl \\ (c)\;CH_3CH_2Cl \\ (d)\;CH_2 =CHCH_2Cl$

Vinyl halides are less reactive than alkyl and halides towards nucleophilic substitution reactions.
So, $(b)\;CH_2 =CHCl$
Hence b is the correct answer.