Explanation

(a)
According to Born-lande equation
$U = \frac{Z^{+}\ Z^{-}e^{2}An}{r_{node}}\left( \frac{1}{n} - 1 \right)$
Where , U is lattice energy
$r_{node}$ is interionic distance
$\because$ $U \propto \frac{1}{interionic\ disance}$
$\therefore$ Ions should be of small size to have high lattice energy.