Which middle level clouds are characterized by rain, snow, or ice pellets that pose a serious icing problem if temperatures are near or below freezing?

Nimbostratus clouds are extensive, thick, and typically cover the sky, often associated with steady precipitation such as rain, snow, or ice pellets. These clouds form in stable atmospheric conditions and are continuous, which means they can produce consistent precipitation over an extended area. When temperatures are near or below freezing, the precipitation falling from nimbostratus clouds can contribute to serious icing conditions, creating hazardous conditions for flight operations.

The other types of clouds mentioned do not consistently produce the same kind of precipitation. Altostratus lenticular clouds are usually associated with smooth, layered clouds that may not generate significant precipitation. Altocumulus castellanus clouds indicate instability and may produce some isolated showers, but they are not generally associated with steady or significant precipitation. Cumulonimbus clouds, while they can produce severe weather, are typically associated with thunderstorm activity rather than the steady, cold-induced precipitation that nimbostratus clouds provide. Therefore, nimbostratus is the best answer as it directly relates to the continuous precipitation that can lead to icing conditions in colder temperatures.