Answer:
Na3PO4
Explanation:
If 1 mol of each of these solutes is added to the water, one that would produce most ions (particles) in the solution will have lowest freezing point. This is because, more number of ions (particles) will need more time and cooler temperature to arrange themselves in a solid form. However, when the number ions (particles) will be less, the solification process would be faster. For the given examples, we can see that
CuCl2 -> Cu(+) + 2Cl(-), would have 3 ions/particles
Na3PO4 -> 3Na(+) + PO4(-), would have 4 ions/particles
C12H22O11 -> C12H22O11, would have 1 particle
NaOH -> Na(+) + OH(-), would have 2 particles
Mg(OH)2 -> Mg(+) + 2(OH(-), would have 3 particles.
PS: See van 't Hoff factor freezing point depression for more details.
