One molar solution of which substance has a maximum O.P.
A. $NaCl$
B. Glucose
C. Fructose
D. Starch

Hint: Osmosis can be defined as the process of movement of solvent molecules from regions of higher concentration to that of lower concentration through a semipermeable membrane.

Complete answer: Osmosis is a passive process that occurs along the concentration gradient, i.e., there is no expenditure of energy taking place in this process.
Osmotic pressure (O.P.) is the maximum pressure that is required to prevent mixing of pure water into the aqueous solution through a semipermeable membrane.
The osmotic pressure can be affected by following factors:
Concentration of solute particles: It is directly proportional to the osmotic pressure, that is, if the concentration of solute particle increases in the solution, then more osmotic pressure should be applied.
Ionisation of solute particle: It is also directly proportional to the osmotic pressure, for instance, addition of $NaCl$ in water solution NaCl hydrolysis to form sodium ion and chloride ion that interacts with $H^{+}$ and $OH^{-}$ ions of water which leads to the interaction between solute and solvent.
Hydration of Solute particles: Hydration is the process in which solute particles are surrounded by water, that means osmotic pressure is directly proportional to hydration of solute particles.
Temperature: Temperature is directly proportional to the osmotic pressure on increasing temperature Kinetic energy of solute and solvent particles increases which result in more movement of water molecules.
As osmotic pressure depends upon the number of solute particles, so maximum O.P. is found in sodium chloride. One molecule of sodium chloride in water will form two solute particles- one sodium ion and one chloride ion. All other molecules i.e.- glucose, fructose and starch remain some particle in water.
So, the correct answer is option A, NaCl.

Note: The maximum osmotic pressure is found in halophytes. The halophytes are the plants that grow in the saline condition. The osmotic pressure of pure water is zero.