We have remember that lattice energy increases as the size of the ions in the ionic solid decreases. According to Oxford dictionary; lattice energy is "a measure of the energy contained in the crystal lattice of a compound, equal to the energy that would be released if the component ions were brought together from infinity."
We know that the larger the magnitude of charge on a cation the smaller in size the cation is and the size of anions increase from left to right in the periodic table. Hence Ca^2+<Na^+ and O^2- < Cl^-. Therefore the lattice energy of CaO is greater than that of NaCl.
the answer is c. their atomic masses are different clearly because an atom of gold has 79 protons and the atom can be divided multiple times. An atom of silver has an atomic number of 47. 47 electrons. Clearly different. Hope it helps :)
