Which of the following is the process of circulating air throughout a substance?

Aeration is the process that involves circulating air throughout a substance, particularly in contexts such as soil or compost. This process is essential because it enhances the availability of oxygen, which is vital for microbial activity and healthy plant growth. When soil is aerated, it allows for better water infiltration, nutrient absorption, and root development, leading to a more robust ecosystem for plants.

In contrast, the other options pertain to different processes. Watering involves supplying water to plants or soil, which is necessary for hydration but does not specifically refer to the circulation of air. Tilling involves turning over and breaking up soil to improve its conditions for planting but does not directly relate to air circulation. Secretion refers to the process where substances are produced and released by cells or glands in organisms, which is unrelated to the concept of air movement in a substance. Thus, aeration stands out as the correct choice in the context of air circulation.