Final answer:
Gas giant planets, specifically Jupiter and Saturn, primarily have atmospheres composed of hydrogen and helium. These elements were confirmed to be the dominant gases through spectral analysis and are aligned with formation theories of these planets.
Step-by-step explanation:
The gas giants in our solar system have atmospheres that are composed primarily of hydrogen and helium. These include the largest planets, Jupiter and Saturn, which are also known as the 'warm gas giants.' The term 'gas giants' can sometimes include Uranus and Neptune, termed 'ice giants', due to their higher proportion of 'ice' components. However, for the gas giants, the atmospheres are significantly hydrogen-based, and any observed compounds typically are hydrogen-rich, such as methane (CH4) and ammonia (NH3). While the early solar system contained various gases, most of the oxygen combined with hydrogen to form water (H2O), making oxygen relatively scarce in the outer solar system and thereby influencing the chemistry of these gas giant planets.
The correct answer to the question 'Gas giants have atmospheres composed primarily of which elements?' is option a) hydrogen and helium. This composition underlying the planetary formation is supported by calculations and measurements dating back more than half a century. Furthermore, encounters from space missions like Voyager have confirmed the abundance of hydrogen and helium through spectral analysis.