Final answer:
The eyes, skin, and respiratory system are most vulnerable to mustard gas, which could cause severe burns and respiratory damage during exposure, leading to long-term health issues or death.
Step-by-step explanation:
The eyes, skin, and respiratory system are the most vulnerable parts of the body to mustard gas. Mustard gas, which was widely used during World War I, caused a range of severe physical conditions. When exposed, mustard gas could cause burns to the skin, and its vapors could severely damage the eyes and lungs. Protective equipment such as gas masks became critical for soldiers, and failing to don them promptly during an attack could result in injury or death. The use of this gas has been outlawed due to its devastating and long-lasting effects on human health.