What fermentation byproduct can smell like bananas?

The correct choice, which is isoamyl acetate, is a fermentation byproduct that is well-known for its banana-like aroma. This compound is produced during the fermentation process, particularly by certain strains of yeast, especially when fermentation occurs at warmer temperatures. Isoamyl acetate is an ester formed from the reaction of isoamyl alcohol (a type of fusel alcohol) and acetic acid. Its presence contributes significantly to the characteristic fruity and tropical notes in various beer styles, particularly in Hefeweizen and some other wheat beers.

Understanding this compound is crucial for brewers aiming to achieve specific flavor profiles in their beers. While other compounds like diacetyl, acetic acid, and fusel alcohols also appear in fermentation, they do not impart the banana aroma that isoamyl acetate does. Diacetyl is associated with a buttery flavor, acetic acid gives a vinegar-like aroma, and fusel alcohols can add harsh flavors rather than fruity ones. Thus, isoamyl acetate stands out distinctly for its banana scent, making it the right answer for this question.