Which of the following is NOT a group 4 synthetic auxin?

Diuron is the correct choice as it is not classified as a Group 4 synthetic auxin. Group 4 herbicides are known for mimicking the natural plant hormone auxin, which regulates plant growth. This group includes herbicides such as Picloram, Triclopyr, and 2,4-D, all of which act by disrupting normal plant growth processes, leading to uncontrolled growth and eventual death in susceptible plants.

In contrast, Diuron is a type of herbicide that belongs to the urea family and functions differently. Its primary mode of action is the inhibition of photosynthesis in plants, which is a distinct mechanism compared to the growth-regulating activity seen among Group 4 synthetic auxins. This fundamental difference in how these herbicides operate in plants highlights why Diuron is not part of the Group 4 synthetic auxin classification. Understanding the specific mechanisms of action of herbicides is crucial for their effective application in managing unwanted vegetation, especially in right-of-way settings.