Which type of degradation predominantly occurs in the absence of microorganisms?

Chemical degradation is the correct answer because it refers to the breakdown of substances through chemical reactions that do not necessarily involve biological organisms. This form of degradation can occur due to a variety of factors including changes in temperature, pH, or the presence of other chemicals in the environment.

In the absence of microorganisms, chemical degradation can continue to take place as certain herbicides can react chemically with elements in the environment, leading to their breakdown. This process might involve hydrolysis, oxidation, or other reactions that facilitate the degradation of the herbicide.

Other forms of degradation, like microbial action, rely specifically on the activity of microorganisms, and photodegradation relies on sunlight for the breakdown of substances. Natural degradation is a broader term that can encompass various processes, but when the context is focused on the absence of microorganisms, chemical degradation is the most appropriate response.