The size of a raw mango shrinks to a much smaller size when kept in a concentrated salt solution. Which one of the following process can explain this?

We have a situation where a raw mango (which is essentially a cell-filled fruit) is placed in a highly concentrated salt solution. Immediately after contact, water starts moving out of the mango and, with time, the fruit becomes visibly shrunken and wrinkled. To decide which physical process is responsible, we must recall the definitions of the candidate processes.

Osmosis is defined as the spontaneous passage of solvent molecules (generally water) through a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration. Mathematically, the driving force behind osmosis is the osmotic pressure, expressed by the van ’t Hoff relation

$$\pi = iCRT,$$

where $$\pi$$ is the osmotic pressure, $$i$$ is the van ’t Hoff factor, $$C$$ is the molar concentration, $$R$$ is the universal gas constant and $$T$$ is the absolute temperature.

In a plant cell, the cell membrane along with the cell wall behaves like a semi-permeable barrier, permitting water to move while largely restricting the passage of larger solute particles.

Now, when the raw mango is immersed in a concentrated salt solution, the external medium has a much higher solute concentration than the cell sap inside the mango. According to the principle of osmosis, water must therefore travel from the region of lower solute concentration (inside the mango) to the region of higher solute concentration (outside, in the salt solution). As water exits the cells, the volume of the cells decreases, leading to plasmolysis and overall shrinkage of the mango.

Let us contrast this with the other listed processes:

Dialysis involves the separation of small solute particles from colloidal macromolecules through a semi-permeable membrane; it is aimed at solute movement and not primarily solvent flow.

Diffusion is the movement of molecules from higher to lower concentration without any requirement of a semi-permeable membrane. In the present case, the crucial feature is the membrane control, so plain diffusion is not the deciding factor.

Reverse osmosis is the forced movement of solvent from a concentrated to a dilute solution by applying an external pressure greater than the osmotic pressure. Here we are not applying external pressure; the movement is spontaneous.

So, none of these alternate processes match the observed behaviour as precisely as osmosis does.

Hence, the correct answer is Option A.