Which chemical agent is characterized by a dark yellow appearance?

The correct choice is Mustard-Lewisite, HL, which is known for its dark yellow appearance. This chemical agent belongs to the category of blistering agents and is a combination of mustard gas and Lewisite. Mustard gas itself is recognized for its yellow-brown color and can cause severe blistering upon contact with the skin. The dark yellow appearance of HL makes it identifiable in various scenarios, which is crucial for military personnel to recognize in order to take appropriate protective measures.

While VX is a highly toxic nerve agent that is colorless to pale yellow and lacks the distinctive dark yellow characteristic, Sarin is also a nerve agent typically colorless and odorless. Chlorine, on the other hand, is a greenish-yellow gas that does not possess the dark yellow hue attributed to Mustard-Lewisite. Understanding the visual characteristics of chemical agents is essential for identification and response in a military context, highlighting the significance of recognizing Mustard-Lewisite, HL, as the agent with a dark yellow appearance.