Vapor pressure lowering
The answer hings on the colligative properties of solutes. In essence solutes lower the vapor pressure of the solvent. NaCl splits into two ions (i.e. two solute "particles") and lowers the vapor pressure of water more than urea for which the whole molecule dissolves as one solute "particle." 

If there was no liquid the Hg level would be higher than any of the three tubes shown since there would be a pure vacuum above the Hg. 

The scale of the relative differences in drawing is obviously greatly exaggerated.