The temperature at which vapor pressure of liquid becomes equal to the atmospheric pressure is called the boiling point of the liquid.
Boiling points of hydrocarbons like alkanes are affected by several factors.
For alkanes, the boiling point increases as molecular mass increases. This is because the intermolecular Van der Waals force increases as molecular size increases.
Hexane, 2 -methylpentane, 3 -methylpentane, and 2,2 -dimethylbutane have the same molecular formula but different structural formula. Such compounds are called structural isomers of each other. These structures differ in the length of continuous parent carbon chains. The structures are as follows-

For isomeric alkanes, we observe an interesting pattern in the boiling point. In case of isomeric alkanes, the boiling point decreases as branching in the molecule increases. This is because as the number of branched chains in the molecule increases it gains the shape like a sphere. Now because of the spherical shape, the molecule has less area of contact and therefore the intermolecular force between the spherical molecules becomes weak. Now, these weak intermolecular forces can be overcome at relatively low temperatures.
In the above structures, we can see that 2,2 -dimethylbutane has the highest branching as compared to the other structures. Hence it will have the lowest boiling point.
Hence, the correct option is (D).