Final answer:
A positive phase of the Arctic Oscillation is associated with mild conditions in Europe and the Eastern US, as it involves the trapping of cold air at higher latitudes.
Step-by-step explanation:
The correct answer to the question of which phase of the Arctic Oscillation (AO) is typically associated with mild conditions in Europe and the Eastern US is A. Positive. During the positive phase of the AO, low-pressure systems in the Arctic lead to cold air being trapped at higher latitudes, and warmer air tends to dominate in regions like Europe and the Eastern United States. Conversely, the negative phase of AO usually brings colder conditions to these areas due to the southward movement of cold polar air.