(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of (A).
Ethanol is a covalent compound with weak intermolecular forces, giving it low melting and boiling points. Ionic compounds like NaCl have very strong inter-ionic forces, hence high melting and boiling points.
The textbook states: "Covalently bonded molecules have strong bonds within the molecule, but inter-molecular forces are weak — giving rise to low melting and boiling points." Ionic compounds (NaCl) have strong electrostatic forces between ions, so their melting/boiling points are much higher. The Reason directly explains why the Assertion is true, making option (a) correct.