Which of the following gases harms beer flavor and aroma?

Oxygen is a gas that significantly harms beer flavor and aroma. When oxygen is introduced to beer, it can lead to a variety of undesirable changes, often referred to as oxidation. This process can result in off-flavors and aromas such as cardboard, paper, or sherry-like characteristics, which detract from the intended taste profile of the beer.

In brewing and beer storage, exposure to oxygen is something that brewers strive to minimize. Brewers take careful measures to protect the beer from oxygen during the brewing process as well as during packaging and storage, as even small amounts can lead to significant flavor degradation over time.

Other gases like carbon dioxide and nitrogen are commonly used in the brewing and serving process and actually contribute positively. Carbon dioxide is essential for carbonation, providing that desirable fizziness, while nitrogen is used in certain styles of beer, such as stouts, to create a smoother mouthfeel and a creamy head. Hydrogen, while generally inert, does not have a direct influence on the flavor or aroma of beer. Therefore, oxygen stands out as the gas that poses a threat to the integrity of beer's flavor and aroma.