Answer: option 4) NaCl
Step-by-step explanation:
The boiling point of a solvent increases as the number of dissolved particles of solute increase.
The formula is:
ΔTb = i * m * Kb.
Where i is the van't Hoff constant, m is the molality of the soluton and Kb is the molal boiling constant.
i accounts for the dissociation of the particles, the higher the number of ions a compound yields in solution the hiigher the i factor and the higher the increase in the boiling point.
The ionic compound Mg Cl2 dissociate into 1 ion Mg (2+) and 2 ions Cl (-), which is 3 ions.
The compounds FeCl2, CaCl2, and BaCl2, all three, also dissociate into 3 ions each, so there is not difference with MgCl2.
On the other hand, NaCl dissociattes into two ions: Na(+) and Cl(-) which means that this solute will have a lower incidence on the increase of the boiling point, being the answer to the question.