Explanation

(b)
Ionisation energy is the amount of energy required to take out most loosely bonded electron from an isolated gaseous atom. In a group when we move from top to bottom, ionisation energy decreases due to increase in size. In a period while moving from left to right ionisation energy increase due to increase in size. In a period while moving from left to right ionisation energy increase due to increase in size.
$\therefore Be > Mg > Ca$ ($\because$It is the order of increasing ionisation energy when we move from top to bottom in group II A).