Consider the following compounds,

A. n-Butane

B. 2-Methylbutane

C. n-Pentane

D. 2, 2-Dimethylpropane

The correct decreasing order of their boiling points is-

1. A>B>C>D

2. B>C>D>A

3. C>B>D>A

4. D>C>B>A

Hint: Boiling point inversely proportional to branching
Boiling point of an alkane depends on number of carbon atoms, as the number of carbon atom increases boiling point also increases. It also depends on branching in the alkane, as branching increases boiling point of alkane decreases.

Alkanes with even number of carbon atoms have greater melting points than those with odd number of carbon atoms
As per both factor the order of boiling point is as follows:
n-butane < 2,2-dimethylpropane < 2-methylbutane  <  n-pentane

The value of boiling point is as follows: