What type of clouds typically indicate thunderstorms?

Cumulonimbus clouds are the correct answer because they are the primary type of cloud associated with thunderstorms. These towering clouds exhibit significant vertical development and can reach high altitudes, often demonstrating an anvil-shaped top. The characteristics of cumulonimbus clouds include their ability to produce severe weather phenomena, such as heavy rain, lightning, hail, and even tornadoes. Their formation is associated with strong updrafts and instability in the atmosphere, making them a clear indicator of thunderstorm activity.

Cumulus clouds generally appear as fluffy white clouds and can grow into larger cloud types, but they do not specifically indicate thunderstorms. Stratus clouds are layered and typically bring overcast skies with light precipitation rather than severe weather. Nimbostratus clouds produce steady, continuous rain, but they lack the vertical development and potential for severe weather associated with thunderstorms.