The correct answer is B) Cumulonimbus. The classic "thunderhead" cloud is the cumulonimbus cloud, which is characterized by its towering, anvil-shaped appearance and is associated with thunderstorms. Cumulonimbus clouds can reach high altitudes and are capable of producing heavy rain, lightning, hail, and strong winds, which are the hallmarks of a thunderstorm. The other answer choices are incorrect because: A) Cirrus clouds are high-altitude, wispy clouds; C) Altostratus clouds are mid-level, sheet-like clouds; and D) Stratus clouds are low-lying, flat clouds that do not have the distinctive towering appearance of a cumulonimbus cloud.