What condition causes the red [CKPT PX] annunciator to illuminate?

The correct choice indicates that the red [CKPT PX] annunciator illuminates when cockpit differential pressure exceeds 3.9±0.1 psi. This condition is critical because it can indicate a potential pressure-related issue in the cockpit environment. Cockpit differential pressure is essential for maintaining a safe and comfortable atmosphere for the pilots, and exceeding this threshold could suggest over-pressurization, which can lead to structural issues within the cockpit or complications related to air supply.

Understanding the importance of monitoring cockpit pressure ensures that flight crews can take timely action to address any anomalies, thereby enhancing safety in flight operations. The illumination of this annunciator serves as an immediate warning that requires attention and assessment of the cockpit's pressurization system.