Based on Mark's decision not to take his umbrella, the cloud he most likely saw in the sky is the **cumulonimbus** cloud. Cumulonimbus clouds are tall, dense, and associated with thunderstorms and heavy rainfall. When Mark saw this type of cloud, he probably assumed that there would be no need for an umbrella as it indicates the possibility of rain.
On the other hand, **cirrus** clouds are thin, wispy clouds found at high altitudes and do not typically signal immediate precipitation. **Nimbostratus** clouds are thick, dark clouds that bring steady precipitation but may not necessarily be associated with thunderstorms like cumulonimbus clouds.
Therefore, the presence of cumulonimbus clouds in the sky would be the most likely reason for Mark's decision not to take his umbrella.
