Which type of cloud is an indicator of potentially severe weather?

Cumulonimbus clouds are the correct choice as an indicator of potentially severe weather because they are towering, large clouds that can produce significant weather phenomena, including thunderstorms, heavy rain, hail, and tornadoes. These clouds are characterized by their vertical development, often reaching high into the atmosphere, indicating strong convection processes at play. The presence of cumulus clouds does not necessarily imply severe weather, as they often represent fair weather conditions. Cirrus clouds are high-altitude clouds that typically indicate fair weather but can also signal changes in the weather, although they are not directly associated with severe weather. Nimbostratus clouds are thick clouds that produce steady rain or snow but are generally associated with prolonged, steady precipitation rather than severe weather conditions. Therefore, cumulonimbus clouds are specifically linked to the more extreme aspects of weather, making them a crucial focus for anyone monitoring weather conditions for sailing or flying.