Bitterness in wine may come from...

Bitterness in wine is primarily associated with tannins, which are natural compounds found in grape skins, seeds, and stems, as well as in oak barrels used for aging. Tannins can impart a drying, astringent sensation that can be perceived as bitterness, especially when they are present in higher concentrations, which is often the case in wines that have been aged in new oak barrels. These barrels can impart additional tannins to the wine, enhancing the bitterness if not balanced with other components.

The other choices do not directly contribute to the bitterness in wine in the same manner. Old oak barrels typically impart subtle flavors and compounds without significantly adding bitterness, as they have already released most of their tannins during previous uses. Sweet wines typically mask bitterness with sweetness, so they are less likely to exhibit bitterness. High acidity, while it can create sharp flavors, does not contribute to bitterness itself; instead, it brings freshness and balance to the wine. Therefore, the association of bitterness with tannins and new oak is the correct understanding in this context.