The correct answer is (d), Sarin gas. Sarin is a deadly nerve agent that was used as a chemical weapon in the Syrian Civil War. It is a colorless, odorless liquid that can be inhaled, ingested, or absorbed through the skin. Sarin causes rapid death by paralyzing the muscles that control breathing and heart function.
Mustard gas is a blistering agent that was used as a chemical weapon in World War I. It is a yellow-brown liquid that can cause severe burns to the skin, eyes, and lungs. Chlorine gas is a greenish-yellow gas that is used as a disinfectant and bleaching agent. It can be fatal if inhaled in high concentrations. Methyl isocyanide gas is a highly toxic gas that was used in the Bhopal disaster in 1984. It causes respiratory failure and death.
Sarin gas is a much more deadly weapon than mustard gas, chlorine gas, or methyl isocyanide gas. It is also much more difficult to detect and protect against. This is why it was used as a chemical weapon in the Syrian Civil War.