This is a trick question really, or at least the wording is poor chemistry. Molecules are the smallest unit of a simple covalent compound that retain the chemical properties of that compound. Covalent means there are bonds between atoms due to sharing a pair of electrons, and an example of such a compound is water, H2O. There are discrete H2O molecules moving past each other, with only weak intermolecular forces in between them.
Yet MgCl2 is not a covalent compound. In fact the bonding is ionic, meaning that electrons are transferred between the atoms to form positive and negative ions, which then electrostatically attract. Ionic compounds form a giant ionic lattice, a continuous structure of millions of these ions. An ionic molecule does not really exist.
What I assume is meant is the formula unit, a term very similar in meaning to a molecule. The difference is well shown by comparing empirical formula and molecular formula. A molecular formula shows how many of each atom are in a molecule, yet the empirical formula just shows the simplest whole-number ratio between the number of these atoms. Take ethane: the molecular formula is C2H6 and the empirical formula is CH3.
The ionic formula unit is like this empirical formula. By this standard the likely answer is A (1), because the formula shows that for every magnesium atom, there are two chlorine atoms.
I hope this helps :)
