Any country capable of producing organophosphorus pesticides can also produce which chemical agent?

The correct answer revolves around the chemical similarities between organophosphorus pesticides and certain chemical agents. Organophosphorus compounds are widely used in agriculture and can also be precursors for the synthesis of nerve agents. Lewisite, specifically, is a chemical warfare agent known for its blistering effects and is classified as a type of arsenic-based compound, but its production can leverage similar chemical synthesis techniques used for organophosphates.

The common feature in the production process of both organophosphorus pesticides and Lewisite lies in their chemical structures, particularly the phosphorus atom's role in both functionalities. This shared chemistry enables a country with the capability to produce organophosphorus pesticides to also have the technical expertise and equipment necessary to manufacture Lewisite. This correlation highlights the dual-use nature of chemical production and the potential for dual-use technologies in the realm of chemical weapons and agricultural chemicals.

In contrast, the other options—chlorine, mustard gas, and phosgene—do not share this direct relationship. Chlorine is an elemental gas with a standalone production process, and the other two agents involve different chemical compositions and methodologies that are distinct from organophosphorus compounds, thus making them less relevant in this context.