Final answer:
Nitrogen dioxide (NO₂) is the pollutant among the options provided that is most likely to make breathing problems worse, due to its role as a primary respiratory irritant.
Step-by-step explanation:
The pollutant likely to make breathing problems worse among the options provided is Nitrogen dioxide (NO₂). Nitrogen dioxide is a highly reactive gas that contributes to smog and acid rain, causing respiratory issues such as eye, nose, and throat irritation, shortness of breath, and an increased risk of respiratory infection. Breathing in NO₂ can exacerbate asthma and other lung conditions, making it particularly harmful for individuals with existing breathing problems. Other pollutants like carbon monoxide (CO), ground-level ozone (O₃), and particulate matter (PM) also have significant effects on respiratory health, but NO₂ is known for being a primary irritant to the respiratory system.