Answer: A.   pure water
Explanation:
 
 = change in boiling point
 = change in boiling point
i = Van'T Hoff factor  
 = freezing point constant
 = freezing point constant
m = molality
1. For pure water
, i= 1 as it is a non electrolyte and does not dissociate.
2. For  

, i= 2 , as it is a electrolyte and dissociate to give 2 ions and concentration will be (0.5+0.5)=1M[/tex] 
3. For  

, i= 2 , as it is a electrolyte and dissociate to give 2 ions and concentration will be (1+1)=2M[/tex] 
4. For  

, i= 2 , as it is a electrolyte and dissociate to give 2 ions and concentration will be (2+2)=4M[/tex] 
Thus as vant hoff factor is lowest for pure water and thus the boiling point will be lowest.