Which of the following actions is prohibited when performing aerobatics?

Creating a fuel imbalance over 50 pounds during aerobatics is prohibited because it can significantly affect the aircraft's center of gravity and overall stability. A fuel imbalance of this magnitude could lead to unpredictable handling characteristics, especially during maneuvers that require precise control, such as aerobatics. When performing aerobatic maneuvers, the aircraft must be in a specific performance envelope, and significant weight shifts can compromise safety and performance.

In contrast, using partial power, applying flaps, or flying with the engine in idle may have specific operational considerations, but they do not inherently pose the same level of risk associated with maintaining a balanced airframe during aerobatic maneuvers. The other actions might be limited in specific contexts, but the fuel imbalance rule is particularly critical for maintaining the structural and control integrity of the aircraft during high-stress maneuvers.