The correct answer is C) probable thunderstorm activity. Cumulus clouds that have undergone vertical development and have become cumulonimbus in form indicate the likely presence of thunderstorm activity. Cumulonimbus clouds are characterized by their towering, anvil-like appearance and are associated with the formation of thunderstorms due to the intense updrafts of warm, moist air. This atmospheric condition is a clear sign of unstable weather and the potential for thunderstorm development, which is the correct explanation for the given scenario. The other options are incorrect because: A) an approaching hurricane or typhoon would typically be indicated by other weather patterns and cloud formations, not just cumulonimbus clouds; B) clearing weather would not be associated with the development of cumulonimbus clouds; and D) the passage of a warm front is not the primary reason for the formation of cumulonimbus clouds in this case.