Final answer:
The necessary in-flight condition for the structural icing to form is visible moisture.
Step-by-step explanation:
One in-flight condition necessary for structural icing to form is A. visible moisture. When there is visible moisture in the air, such as clouds, fog, or precipitation, water vapor can condense and freeze on the aircraft's surfaces, causing structural icing. This can be hazardous as it affects the aerodynamics and stability of the aircraft.