Among arsenic, copper, chlorine, and sulfur, which is used in making synthetic pesticides?

Chlorine is significant in the production of synthetic pesticides primarily due to its reactive nature and role in chemical synthesis. It is frequently utilized in various synthetic compounds, including chlorinated hydrocarbons, which are a class of synthetic pesticides. These compounds are known for their effectiveness against a range of pests and are fundamental in commercial pesticide formulations. Chlorine's ability to engage in chemical reactions makes it a valuable building block in the creation of effective pest control agents.

While the other options have applications in agriculture—such as copper being used as a fungicide, arsenic historically used as a pesticide (now largely restricted due to toxicity), and sulfur being a natural fungicide and insecticide—they do not represent the process of making synthetic pesticides in the same way that chlorine does. Chlorine's distinct chemical properties help create a variety of pesticide compounds that are widely used in modern agriculture.