One in-flight condition necessary for structural icing to form is:

The presence of visible moisture is a critical factor for the formation of structural icing during flight. When an aircraft flies through areas where there is visible moisture, such as clouds, rain, or mist, the moisture can freeze upon contact with the aircraft's surfaces if the temperature is at or below freezing. This freezing can lead to ice accumulation, which can adversely affect aircraft performance by increasing weight, disrupting airflow over the wings, and affecting control surfaces.

In the context of the other choices, clouds with a base above 10,000 feet and warm temperatures near the surface do not necessarily contribute to icing conditions, as icing typically occurs in lower temperatures and within clouds or precipitation where moisture is present. Clear skies typically indicate a lack of moisture, thus reducing the likelihood of icing. Therefore, the presence of visible moisture is indeed a fundamental requirement for structural icing to develop during flight.