Final answer:
During the Ice Age, lower sea levels exposed a land bridge called Beringia, enabling human migration from Asia to the Americas. Glaciers likely prevented southward migration until paths opened, and coastal migrations by water were possible if challenging.
Step-by-step explanation:
During the last Ice Age, the Americas saw significant human migration, which scholars believe occurred approximately 18,000 years ago. This migration was made possible by the occurrence of lower sea levels, which exposed a land bridge known as Beringia between modern-day Alaska and Russia, facilitating migrations from Asia into Alaska. As the climate warmed and glaciers retreated, various human groups began exploring further south into the Americas, crafting new pathways and adapting to different environments.
Answering the student's question, option b, 'Probably prevented southward migration over land until a pathway was cleared across the area of modern Alberta', is the most accurate. The extensive glaciers of the Ice Age created a barrier to southward land migration until a habitable corridor became accessible.
It is also worth noting that, while migration over water was more challenging during the Ice Age, it was not entirely prevented as evidence suggests that some groups did use rafts or canoes for coastal travel. Therefore, option c, which states that migration over water was prevented by ice, is incorrect. Moreover, there is a great diversity in the languages and genetics among the early inhabitants of the Americas, contradicting option d.