Which middle level clouds are known for posing a serious icing problem if temperatures are near or below freezing?

Nimbostratus clouds are the correct choice because they are thick, moisture-laden clouds that often extend over a large area, producing continuous precipitation. When temperatures are around or below freezing, the precipitation from nimbostratus clouds usually falls as freezing rain or ice pellets, which can create hazardous conditions for aviation. The cloud's composition allows for supercooled water droplets to exist, which can lead to significant icing on aircraft. This icing can occur because the droplets do not freeze upon contact with surfaces, creating a dangerous layer of ice.

The other options do not present the same level of icing risk: cumulus clouds are typically associated with fair weather and may not produce significant amounts of moisture; altostratus clouds usually carry less moisture and significant icing is not commonly a problem; while stratus clouds are more uniform and light but do not generally contribute to serious icing conditions either.