Which of the following is NOT prohibited when operating the aircraft?

The selected answer is accurate because performing normal takeoffs is not prohibited when operating the aircraft. Normal takeoffs are essential for flight operation and are executed under standard procedures outlined in the T-6A Operating Limits. These procedures are designed to ensure safety and efficiency during takeoff.

In contrast, other options delineate actions that could compromise safety or the aircraft's integrity. Leaving the PMU off during spins can lead to uncontrolled behavior of the engine and propulsion system, which is critical during such maneuvers. Engaging in aerobatic maneuvers with over a 50-pound fuel imbalance can create an unsafe condition, as the imbalance affects the aircraft’s center of gravity and handling characteristics. Making abrupt cross-controlled maneuvers can lead to loss of control and is generally discouraged in flight operations.

Therefore, while the other actions are prohibited due to the associated risks, performing normal takeoffs is an intended function of the aircraft and a regular part of its operational limits.