Final answer:
During the middle Cretaceous period around 105 million years ago, the significant geological event was the breakup of Gondwana, related to continental drift.
Step-by-step explanation:
The geological event that occurred during the middle Cretaceous period around 105 million years ago was the breakup of Gondwana. Option b) is the correct answer. This event is associated with continental drift, which played a crucial role in the distribution of landmasses on Earth. While the other options like the formation of the Himalayan Mountains and the eruption of the Yellowstone supervolcano are significant geological events, they did not occur in the middle Cretaceous.
The formation of the Himalayan Mountains started mainly after the Indian Plate collided with the Eurasian Plate about 50 million years ago, which is after the Cretaceous period. The Yellowstone supervolcano's caldera-forming eruption occurred much later as well. The Rocky Mountains began their formation in the Late Mesozoic and continued into the Early Cenozoic, but their significant growth occurred after the middle Cretaceous.