The compound having highest boiling point among the following is-

The boiling points of alkanes increase with increase in molecular mass and for the same alkane, the boiling point decreases with branching. In the given question, all have the same number of C atoms but option 2 and 4 are branched compounds, hence, they will definitely have lower boiling point than butane and octane which are straight chain. Octane is a straight-chain compound that has a large surface area than butane. So, there are more van der Waals forces of attraction resulting in a high boiling point.