The aqueous solution that has the lowest vapour pressure at a given temperature is:

0.1 molal sodium phosphate

The correct answer is option 1. 0.1 molal sodium phosphate.

The same concentrations of relative lowering of vapour pressure depend upon the number of ions or particles present in different solutions.

\(Na_3PO_4\) ⇌  \(3Na^+  + PO_4^{3-}\) \( = 4\text{ ions}\) 

\(BaCl_2\) ⇌  \(Ba^{2+}  +  2Cl^-\) \( = 3\text{ ions}\)

\(NaCl\) ⇌  \(Na^+  +  Cl^-\) \( = 2\text{ ions}\)

\(Glucose \) ⇌  \(\text{no dissociation}\) \( = 1\text{ ion}\)

As \(Na_3PO_4\) gives maximum ions (4) on dissociation, hence relative lowering of vapour pressure is maximum for sodium phosphate \(Na_3PO_4\) and it has the lowest vapour pressure.