Answer: Option (C) is the correct answer.
Explanation:
Weak forces of attraction between the molecules of a substance are known as Vander waal forces.
Down the group there is increase in atomic size of the elements of a group due to the increase in atomic mass which increases the number of electrons in the atoms.
These electrons are held by Vander waal forces (or intermolecular forces) and strength of these forces increase down the group. As a result, more heat is required to move the electrons due to which there occurs increase in boiling point of atoms.
Thus, we can conclude that the statement total number of electrons increases down the group, and this increases the strength of the intermolecular forces, best explains the trends in boiling points.
