Why should pilots avoid relying solely on a magnetic compass during turns?

Relying solely on a magnetic compass during turns is problematic primarily due to magnetic variations that can introduce inaccuracies in the readings. A magnetic compass functions based on Earth's magnetic field; however, this field isn't uniform everywhere.

As a result, the compass can be affected by changes in magnetic declination (the angle difference between magnetic north and true north) and local magnetic anomalies. When an aircraft turns, especially if the turn is sharp or if there are significant variations in the local magnetic field, the compass may not align accurately with the aircraft’s true heading. This can cause the pilot to misinterpret their actual direction, especially in a dynamic situation such as turning or maneuvering, leading to potential navigational errors.

The other options, while they do touch on important aspects of flight instrumentation and navigation, don't address the specific and critical concern regarding the reliability of a magnetic compass in turns as thoroughly as the selected answer does. Thus, understanding the impact of magnetic variations is essential for pilots to ensure accurate navigation and operational safety.