In the rainy season, doors get swelled up due to

The correct option is Imbibition.

Here is the step-by-step explanation of why this is the correct answer:

1. Understanding Imbibition:
Imbibition is a special type of diffusion in which water or another liquid is absorbed by solid particles (colloids), causing them to enormously increase in volume. In this process, the water molecules get tightly bound to the hydrophilic (water-loving) surfaces of the solid material without forming a solution.

2. Application to Wooden Doors:
Wooden doors and windows are made of wood, which contains cell wall components like cellulose and pectin. Cellulose is highly hydrophilic, meaning it has a strong affinity for water. During the rainy season, the humidity levels in the air are very high. The wood of the door acts as an imbibant, absorbing moisture directly from the humid air.

3. Swelling and Pressure:
As the hydrophilic colloids in the wood absorb water molecules, they swell, and a significant amount of pressure—called imbibition pressure—is generated. This causes the overall volume of the door to increase, resulting in the door getting swelled up and becoming tight or hard to close.

4. Why other options are incorrect:
- Transpiration: This is the loss of water vapor from the aerial parts of living plants (mostly leaves), which is a biological process not applicable to dead wood.
- Diffusion: While imbibition is a subtype of diffusion, diffusion on its own refers to the general movement of particles from a region of higher concentration to lower concentration. Imbibition specifically accounts for the absorption of liquid by solids leading to swelling.
- Respiration: This is a cellular metabolic process where living organisms release energy from organic molecules, which does not take place in wooden doors.