Which exposure would likely lead to immediate respiratory distress?

Chlorine is a potent irritant to the respiratory system and exposure to it can lead to immediate respiratory distress. Chlorine gas, when inhaled, can cause bronchoconstriction, irritation of the airways, and fluid accumulation in the lungs, resulting in symptoms such as coughing, shortness of breath, and chest tightness. The rapid onset of these symptoms can occur within minutes of exposure, making it a critical situation that requires prompt medical attention.

Carbon dioxide, while it can lead to respiratory distress when present in high concentrations, typically does not cause immediate symptoms unless it reaches dangerously high levels. Ricin is a highly toxic compound derived from castor beans, and while it is dangerous, respiratory distress would not be immediate for most routes of exposure, as it primarily causes systemic effects. Tetanus is a bacterial infection that affects the nervous system and does not cause immediate respiratory distress upon exposure. Therefore, chlorine is clearly the agent that would lead to immediate respiratory problems.