Answer:
a. a land bridge formed between Asia and America
Step-by-step explanation:
Climate Change allowed the biggest human migration. Due to climate change, a land bridge called the Beringia land bridge was formed between Asia and America, that helped Paleolithic hunter-gatherers to enter North America from the North Asian via foot or mammoth.
The formation of land bridge was due to the lowering of sea level during the Last Glacial Maximum that caused biggest human migration.
Hence, the correct answer is "a. a land bridge formed between Asia and America".