Which of the following describes the movement of water vapor towards one or both outer surfaces of the material?

The correct answer is diffusion, as it specifically refers to the natural movement of water vapor from areas of higher concentration to areas of lower concentration. In the context of drying materials, water vapor can migrate through the material and eventually move toward the outer surfaces, where it may evaporate into the air. This process is crucial in drying scenarios, as it helps to diminish moisture content in the materials being dried.

Diffusion is driven primarily by differences in vapor pressure and is influenced by temperature, air movement, and the material's characteristics. Understanding diffusion is key in applied structural drying, as it allows professionals to effectively manage moisture and enhance drying strategies.

Other processes like capillary action involve the movement of liquid water through porous materials due to surface tension and adhesion, rather than the vapor phase. Absorption pertains to the uptake of moisture into a material, while permeation involves the passage of moisture through a material, which could involve both liquid and vapor phases but does not specifically describe the vapor movement toward outer surfaces the way diffusion does.