Which of the following contributes to the formation of ground-level ozone?

Ground-level ozone is not emitted directly into the air but forms through complex chemical reactions in the atmosphere involving precursors. Nitrogen oxides (NOx) and hydrocarbons (often referred to as volatile organic compounds, or VOCs) are key contributors to the formation of ground-level ozone.

When sunlight hits these precursors, they react in a series of photochemical reactions to produce ozone at ground level. The presence of nitrogen oxides usually comes from sources such as vehicle emissions, while hydrocarbons can come from both natural sources, like vegetation, and human activities, such as industrial processes and gasoline vapors.

This understanding makes nitrogen oxides and hydrocarbons the correct choice in this context. Other options, while they may be pollutants with harmful effects, do not play a direct role in the formation of ground-level ozone through these specific chemical processes.