Question

out of the 2 methyl pentane and 2, 3 dimethylpentane which has greater boiling point and why

Answer 1

B has more branching and this means that molecules of B pack less efficiently and so there are fewer Van der Waals forces between
them and so B has the lowest boiling point. On the other hand A is a linear molecule and so packs efficiently, has stronger VdWs forces and so has the highest boiling point. As @DavePhD's data shows B<C<A

Hope it will help you,,