Which of the following is the most probable cause if, while underway, the main propulsion gas turbine air intake blow-in doors open?

Correct Answer: Option D — Ice has formed on the air intakes.

Blow-in doors are designed to open when intake pressure drops below a set threshold, usually due to a blockage. Ice accumulation on the intake screens restricts airflow, creating a significant pressure drop across the intake system, which triggers the doors to open to prevent compressor starvation and surge.