Why do isomeric branched alkanes normally exhibit lower boiling points than unbranched alkanes of the same carbon content?

The correct option is B

Branched alkanes have greater Van der Waal forces

When we compare isomeric alkanes - unbranched and branched - the unbranched alkanes have a greater surface area in terms of the shape of the molecule. This leads to greater van der Waals forces in the molecules of the unbranched alkanes as compared to branched isomers.