As beer ages, which of the following flavors is most likely to develop or increase as a result of oxidation?

As beer ages, oxidation plays a significant role in altering its flavor profile. One of the most characteristic flavors that develops from oxidation is the stale or cardboard-like note. This occurs because oxygen interacts with various compounds in the beer, leading to the breakdown of certain flavor components and the formation of undesirable compounds, such as trans-2-nonenal, which is responsible for those "cardboard" or "paper" flavors.

In contrast, options like fruitiness, caramel, and spicy notes may be influenced by other factors such as the specific ingredients used in brewing or fermentation processes, but they don't typically result from the aging process. Fruitiness may fade, caramel flavors may remain stable or even intensify in some beers, and spicy notes are usually attributed to hop varieties or yeast character rather than oxidation. Therefore, the development of stale, cardboard flavors is a clear indicator of the negative effects of oxidation in aging beer, making it the correct answer in this context.