The correct answer is option 2, surface tension. When a shaving brush is removed from water, the hair of the brush clings together due to surface tension. Surface tension is the phenomenon that occurs at the surface of a liquid, where the molecules at the surface experience stronger intermolecular forces compared to the molecules in the bulk of the liquid. This results in the surface acting like a stretched membrane.

In the case of the shaving brush, the water molecules adhere to the bristles of the brush due to surface tension. As the brush is removed from water, the surface tension pulls the water molecules along with it, causing the bristles to cling together. This is why the hair of the shaving brush sticks together when it is taken out of water.

Viscosity, option 1, refers to the resistance of a fluid to flow. While viscosity may play a role in the behavior of the bristles, it is not the primary reason for the clinging together of the hair.

Friction, option 3, refers to the resistance encountered when two surfaces slide or attempt to slide past each other. While friction may contribute to the behavior of the hair, it is not the main reason for the clinging together.

Elasticity, option 4, refers to the property of a