Final answer:
When the tap is partially open, the beer will pour slowly due to the decreased size of the opening leading to increased resistance against the flow, which is explained by Bernoulli's principle.
Step-by-step explanation:
When a tap is partially open, beer will pour slowly due to the reduced rate of liquid flow. This phenomenon occurs because the opening through which the beer must pass is smaller when the tap is not fully opened, therefore offering greater resistance to the flow of liquid. This effect can be explained by Bernoulli's principle, which states that as the speed of a fluid increases, the pressure within the fluid decreases. When the tap is only partially open, the beer is forced through a narrow section which limits its speed. More foam can also be a result if the beer pours out with turbulence, but given the options presented, slowly is the most appropriate answer.