Final answer:
Oxygen undergoes a chemical change option (b) when it reacts with hydrogen gas to form water vapor, which is a different substance with new chemical properties.
Step-by-step explanation:
When a 2.0-liter volume of hydrogen gas is combined with 1.0 liter of oxygen gas to produce 2.0 liters of water vapor, the oxygen undergoes a chemical change. This is evident because the oxygen and hydrogen gas react to form water vapor, a substance with different chemical properties than the original gases.
This reaction can be represented by the chemical equation 2H2(g) + O2(g) → 2H2O(g). The transformation from oxygen gas to water vapor signifies that a chemical reaction has taken place, changing the composition of the original substances.