An ion has very high charge and very high lattice energy. What do you think can be the size of this ion?
Smaller the size of ions more would be the charge on ions and higher will be the lattice energy.
There are 4 ions P, Q, R & S. P has the highest charge and the highest lattice energy among the given ions. What do you think can probably be the size of P?
When an object has very high resistance, it can be called a good conductor.