Boiling point order of n-pentane (A), 2-methyl butane (B) and 2,2-Dimethyl propane (C) is

In isomeric alkanes,as branching increases,surface  area decreases,hence vander waal's forces of attractions decreases,i.e.,inter molecular forces of attractions decreases,hence the energy required to break such attractive forces decreases.Therefore boiling point and melting point decreases.