Which of the following isomer of pentane $\left({\mathrm{C}}_5{\mathrm{H}}_{12}\right)$ will give three isomeric monochlorides on photochemical chlorination?(stuctural isomers only)

n - pentane on mono chlorination forms :

1-chloro  pentane

2-chloro pentane (optically active )

3 -chloro pentane

including stereo isomers 4 isoers are possible. 

excluding stereo isomerism 3 isomers are possible which are structural isomers