Which of the following is a method of classifying herbicides?

The method of classifying herbicides based on absorption characteristics is correct because absorption refers to how plants take up herbicides through their leaves, roots, or stems. This classification is crucial for understanding how effective a herbicide will be on a specific target plant, as it influences the rate and manner in which the herbicide works within the plant's physiological processes.

Each herbicide offers different modes of absorption, which can greatly affect its efficacy in controlling various types of vegetation. By knowing the absorption characteristics, applicators can better choose the right herbicide for specific conditions, ensuring effective pest management.

The other options, while related to herbicides and their application, do not serve as primary methods for classifying them. For instance, the extent of soil penetration focuses more on the herbicide's movement and effectiveness in the soil rather than how it is absorbed by plants. Duration of effectiveness refers to how long the herbicide remains active before degradation and does not classify the herbicide itself. Exposure to sunlight is related to photodegradation and can affect herbicide stability, but it does not categorize the herbicide based on its modes of action or application. Thus, absorption characteristics stand out as a key criterion in herbicide classification.