What type of substance can pass through the pores of animals and vegetable membranes?

The correct answer is crystalloids. Crystalloids are substances that are capable of passing through semi-permeable membranes, which is a key characteristic when discussing their behavior in biological contexts. These are typically small solute particles that dissolve in a solvent and can easily permeate membranes, allowing for the movement of water and solutes in biological systems. This property is crucial in processes like osmosis and diffusion, where substances move across membranes in response to concentration gradients.

Crystalloids are contrasted with colloids, which consist of larger particles that do not pass through these membranes as readily. Emulsions, which consist of mixtures of water and oils or fats, also cannot pass through in the same way that crystalloids can. Solvents, while they may assist in dissolving crystalloids, do not specifically refer to the particles that cross membranes. Thus, the definition and functional behavior of crystalloids clearly establish them as the correct choice in this context.