What characteristic defines biodegradable substances?

Biodegradable substances are characterized by their ability to be broken down by natural biological processes, primarily through the action of microorganisms such as bacteria, fungi, and other decomposers. These organisms can metabolize organic materials, leading to the breakdown of complex compounds into simpler ones, which can then be reintegrated into the ecosystem as nutrients. This process contributes to nutrient cycling and minimizes the accumulation of waste in the environment, making biodegradable materials environmentally friendly.

In contrast, the other options describe characteristics that do not align with what it means to be biodegradable. Reusable products are designed to be used multiple times but may or may not decompose naturally. Resistance to decomposition for a long period describes non-biodegradable materials, which can accumulate and pose environmental hazards. Lastly, substances that are primarily synthetic often comprise materials that are not biodegradable, as they tend to be designed for durability and longevity, making them challenging for natural processes to break down.