Final answer:
Oxygen is not a significant ingredient of jovian planet atmospheres, which are mainly composed of hydrogen and helium, with smaller amounts of methane and ammonia.
Step-by-step explanation:
The gas that is not a significant ingredient of the jovian planet atmospheres among the options provided is oxygen. The atmospheres of the jovian planets—Jupiter, Saturn, Uranus, and Neptune—are primarily composed of hydrogen and helium, with methane and ammonia also present in smaller quantities. Oxygen, however, tends to combine with hydrogen to form water (H2O) in these environments and is not abundant in the form of free molecular oxygen gas (O2) as it is in Earth's atmosphere.