To solve this problem, we need to identify which of the given options is a secondary pollutant.$\newline$Let's analyze each option:$\newline \bullet$Option 1: SO$_2 \newline$ - Sulfur dioxide (SO$_2$) is a primary pollutant. It is directly emitted from sources such as volcanic eruptions and industrial processes.$\newline \bullet$Option 2: CO$_2 \newline$ - Carbon dioxide (CO$_2$) is also a primary pollutant. It is directly released from processes like respiration and combustion of fossil fuels.$\newline \bullet$Option 3: CO$\newline$ - Carbon monoxide (CO) is a primary pollutant. It is directly emitted from vehicle exhaust and incomplete combustion of carbon-containing fuels.$\newline \bullet$Option 4: O$_3 \newline$ - Ozone (O$_3$) is a secondary pollutant. It is not emitted directly but forms in the atmosphere through chemical reactions between primary pollutants such as nitrogen oxides (NO$_x$) and volatile organic compounds (VOCs) in the presence of sunlight.$\newline$Therefore, the correct answer is Option 4: O$_3$, which is a secondary pollutant.$\newline$This corresponds to the correct option, which is Option 4.