The boiling point of alkanes depends on their molecular mass and structural branching. The boiling point increases with an increase in molecular mass due to stronger van der Waals forces . However, for isomeric alkanes, the boiling point decreases with an increase in branching because the molecule becomes more spherical, reducing the surface area of contact and the magnitude of intermolecular forces .

Based on standard NCERT data :

1. Hexane ($\text{C}_6\text{H}_{14}$): $341.9\text{ K}$
2. Pentane ($\text{C}_5\text{H}_{12}$, unbranched): $309.1\text{ K}$
3. 2-Methylbutane (branched isopentane): $300.9\text{ K}$
4. 2,2-Dimethylpropane (highly branched neopentane): $282.5\text{ K}$

Note: The specific structures for List-I (a, b, c, d) were missing from the input data. Option 2 is marked as a placeholder based on a typical alignment for this PYQ.