Which of the following can cause an aircraft to experience significant icing?

Significant icing can occur within cumulus clouds because these clouds are associated with significant vertical development and moisture. When an aircraft flies through cumulus clouds, it can encounter supercooled liquid water droplets. If the temperature is at or below freezing, these droplets can freeze upon contact with the aircraft's surfaces, leading to the accumulation of ice.

Cumulus clouds often signify unstable atmospheric conditions, which contribute to the formation of these supercooled droplets. The likelihood of icing increases in the colder, higher altitude portions of these clouds, making them particularly hazardous for aircraft.

Thunderclouds can also lead to icing, but the question specifically emphasizes cumulus clouds, which are more broadly indicative of conditions conducive to icing. Thus, while other answers present scenarios that may or may not lead to icing, cumulus clouds distinctly represent an environment where significant icing is likely to occur.