The number of structural isomers which can be obtained theoretically on monochlorination of 2-methylbutane is

It is given to find out the number of structural isomers which can be obtained theoretically on monochlorination of 2-methylbutane. 
Explanation

1-chloro-3-methyl butane i.e. ${\mathrm{CH}}_3-{\mathrm{CHCH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2\mathrm{Cl}$, 2-chloro-2-methyl butane i.e. ${\mathrm{CH}}_3-\mathrm{C}\left(\mathrm{Cl}\right)\left({\mathrm{CH}}_3\right)-{\mathrm{CH}}_2-{\mathrm{CH}}_3$, 1-chloro-2-methyl butane i.e. ${\mathrm{CH}}_2\mathrm{Cl}-\mathrm{CH}\left({\mathrm{CH}}_3\right)-{\mathrm{CH}}_2-{\mathrm{CH}}_3$, 2-chloro-3-methyl butane i.e. ${\mathrm{CH}}_3-{\mathrm{CHCH}}_3-\mathrm{CHCl}-{\mathrm{CH}}_3$ are the four structural isomers that can be theoretically obtained upon monochlorination of 2-methylbutane.
Thus, (D)4 is the correct answer.