Out of the five isomeric hexanes, the isomer that can give two monochlorinated compounds is:

The isomeric hexane should have two different types of H atoms and four similar types of H atoms to give two monochlorinated products.

In 2,3-dimethyl butane, H atoms at positions 'a' and 'b' are the same and at 'b' are the same. So, two positions are available for chlorination.