Cumulus clouds that have undergone vertical development and have become cumulonimbus in form, indicate __________.

Correct Answer: Option C — probable thunderstorm activity

Cumulonimbus clouds represent the mature stage of vertical cloud development. They are characterized by intense updrafts and are associated with heavy precipitation, lightning, and gusty winds. Their presence is a definitive indicator of unstable atmospheric conditions and the high probability of thunderstorm activity in the immediate vicinity.