Which characteristic best explains the difference in melting behavior of covalent substances and ionic substances?
a. ionic substances have weaker intermolecular attractions.
b. ionic substances usually have stronger atomic bonds.
c. covalent substances have weaker intermolecular attractions.
d. covalent substances usually have stronger atomic bonds.

The correct answer is B.  Ionic compounds have stronger atomic bonds.

With ionic compounds, there is a complete transfer of electrons from one atom to another thus creating stronger electrostatic forces between the two atoms.  In regard to covalent compounds, electrons are not transferred but shared between two atoms and the atoms are attracted to each other by weaker electrostatic forces.

 Due to this, it is easier to break covalent bonds than ionic. Thus melting (and boiling) point of ionic compounds is higher than that of covalent compounds.