Phenomena through which water is absorbed by solids such as colloids leading them to increase in volume is

The correct answer is diffusion.

To understand why this is the correct concept, let us break down the physical process step-by-step:
1. Adsorption and Absorption: When water molecules come into contact with solid particles, particularly hydrophilic colloids (like starch, cellulose, or proteins), they adhere to the surfaces and penetrate into the spaces between the particles. This process of uptake is physically driven by the movement of water down a concentration gradient.
2. The Principle of Diffusion: At its core, any net movement of molecules from a region of their higher free energy (or concentration) to a region of their lower free energy (or concentration) is classified as a form of diffusion. When dry solid colloids absorb water, water moves from the liquid phase (high concentration/chemical potential of water) into the dry solid matrix (extremely low chemical potential of water). Therefore, this is fundamentally a special type of diffusion.
3. Increase in Volume: As the water molecules diffuse into the solid substance, they occupy space within the colloidal matrix, causing the solid material to swell and significantly increase in volume.

While this specific type of diffusion where water is adsorbed by solids/colloids is often referred to in botanical contexts as imbibition, imbibition itself is a sub-category or a specialized case of the broader physical phenomenon of diffusion. Thus, diffusion is the fundamental physical process responsible for this behavior.