What environmental change allowed people to travel by land between asia and north america?
a. human adaptation to arctic environments
b. ice sheets blocking sea routes
c. the formation of ice sheets on land
d. the melting of ice sheets on land

D. Beringia i.e the melting of ice sheets on land.

Explanation:

It was possible due to the melting of ice mass that led to the global sea level shift and created a way out for the large scale migration of animals, and the man from Asia to America. Beringia i.e the lands bordering the Machanzi river of Canada and the Chukchi Sea and the Bering Strait in Russia and Alaska. During the ice ages, this area was part of Siberia. And Pleistocene ice age which caused global cooling that leads to the migration of adapted humans some 20,000 years ago.