CaO and NaCl have the same crystal structure and approximately the same ionic radii.

According to the coulomb's law:

F=Kq1q2∕r

Where q1 and q2 are charges on particles and r is the distance between them. 

Since interionic distances in CaO and NaCl are comparable, (bigger cation has more modest anion as well as the other way around).

So, r is practically similar. 

In this manner, lattice energy relies just upon charge. 

Since the charge on Na⁺  and Cl^- particles is unity and that on  Ca²⁺ and O^2- is 2 each,  in this manner the lattice energy of is CaO is  4 times the lattice energy of NaCl, i.e., 4U.

Final answer :   Hence,the correct answer will be option D.4U.