Which of the following substances counts as a congener?

Congeners are substances that are produced during the fermentation process and contribute to the complexity and character of alcoholic beverages. They include various chemical compounds that can influence the flavor, aroma, and overall quality of a spirit. Aldehydes, specifically, are types of congeners that can arise during fermentation and distillation. They often contribute to the taste and scent profile of spirits, adding to both their complexity and potential for varied flavor experiences.

In contrast, water is a primary ingredient in spirits but does not belong to the category of congeners, as it does not provide additional flavors or aromas. Carbon dioxide, while present in some alcoholic beverages and a byproduct of fermentation, is not classified as a congener because it does not contribute to the flavor profile of distilled spirits. Cider is a type of fermented beverage itself and not a specific congener, as it is a finished product rather than a component that derives from the fermentation of sugars into alcohol.

Thus, the identification of aldehydes as congeners is rooted in their role in the sensory characteristics of distilled spirits, significantly impacting the overall drinking experience.