Final answer:
The statements indicating the presence of fossils of ancient sea life on dry land, the existence of many layers of rock, and the evidence of ancient geological activity such as volcanic eruptions and erosion provide strong support for the notion that Lake Baikal is extremely old.
Step-by-step explanation:
The statement that most clearly supports the key idea that Lake Baikal is extremely old can be found in the description of the evidence for an ancient Earth presented in paragraph 4. The evidence includes fossils of ancient sea life found on dry land far from oceans, suggesting that these areas were once covered by sea, revealing a significant passage of geological time. Furthermore, the documentation of many layers of rock, which represent the order in which rocks and fossils appeared, allows scientists to trace Earth's history and by extension, the age of features like Lake Baikal. Lastly, the references to ancient volcanic eruptions, earthquakes, and erosion patterns showcase the Earth's long-term geological activity, implying that the Earth and features like Lake Baikal are extremely old.