Hops added during which phase of brewing primarily influences bitterness?

The boiling phase is critical in the brewing process for developing bitterness in the final beer. When hops are added during boiling, the heat activates the hops' resins, particularly alpha acids, which are responsible for the bitter flavor. The longer the hops are boiled, the more bitterness will be extracted due to the isomerization of these acids.

In contrast, hops added during fermentation (the fermentation phase) tend to impart aroma and flavor rather than bitterness, as the fermentation process does not extract these qualities from hops in the same way that boiling does. The conditioning phase primarily focuses on the maturation of the beer, and while hops can be added for aroma during this time (a process known as dry hopping), it does not contribute to bitterness. The cooling phase is primarily about transitioning the wort to a temperature suitable for fermentation and does not involve significant hop additions that would influence bitterness.

Thus, the boiling phase is essential for any brewer looking to achieve the desired level of bitterness in their beer, making it the correct choice for this question.