(a) Because macromolecules such as proteins and polymers are generally not stable at higher temperature and osmotic pressure is measured at room temperature.
(b) Because O2 is more soluble in cold water at low temperature.
(c) Due to dissociation of KCl as KCI→K++CI−, the number of particles of solute KCl increase, so colligative property (elevation in boiling point) also increases but there is no such dissociation in Sugar solution.