The correct answer is A. It created land bridges where the ocean had once been.
During the last ice age, lower sea levels due to the vast amounts of water trapped in glaciers exposed land bridges, such as the Bering Land Bridge between Asia and North America, which facilitated human migration. While options B, C, and D may relate to adaptations humans made in response to changing environments, the significant migration due to the last ice age is best captured by the creation of land bridges.