If 1 mol of each of these solutes is added to water, which will have the lowest freezing point?

CuCl2
Na3PO4
C12H22O11
NaOH
Mg(OH)2

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.