Which one of the following gases is reported to retard photosynthesis?

Among the given gases, $$\text{NO}_2$$ (nitrogen dioxide) is reported to retard photosynthesis. Nitrogen dioxide reacts with water to form nitrous acid and nitric acid. In the atmosphere, elevated concentrations of $$\text{NO}_2$$ interfere with the photosynthetic process in plants by damaging leaf tissue and inhibiting chlorophyll function.

$$\text{CO}$$ is primarily toxic by binding to hemoglobin. $$\text{CO}_2$$ is actually a reactant in photosynthesis and does not retard it. CFCs (chlorofluorocarbons) deplete the ozone layer but are not directly known to retard photosynthesis. $$\text{NO}_2$$ is specifically documented as a photosynthesis inhibitor because it generates reactive oxygen species and causes oxidative stress in plant cells.

Therefore, the correct answer is $$\text{NO}_2$$.