Given data : It is given to identify Which one of the following compounds most readily undergoes $S_{N}1$ reaction.

Explanation

Tert-alkyl halides, sec-alkyl halides, and primary alkyl halides are the order of increasing nucleophilic substitution by $S_{N}2$ mechanism.
The main alkyl halide in compound B.
Therefore, it is most easily replaced by the $S_{N}2$ mechanism.
Note that although compound A is a primary alkyl halide as well, its connected carbon is more inhibited.

Hence, the correct option is B.