Which of the following is a trace gas?

nitrogen

oxygen

ozone

hydrogen

Hydrogen and ozone are both examples of trace gases.

Explanation:

Trace gases are present in minor quantities in the earth's atmosphere and all the gases except nitrogen, oxygen and argon which make up the major portion of earth's atmosphere are considered trace gases.

The respective percentages of nitrogen, oxygen and argon in the atmosphere are 78.1%, 20.9% and 0.934%. The process that produce trace gases is called source and the process that removes trace gas from the atmosphere is called sink.

Some other examples of trace gases are carbon dioxide, neon, helium etc.