The physical features of eastern europe were shaped by a time of lower temperatures when much of the land was covered in snow and ice known as a(n)
a. stone age.
b. ice age.
c. emigration.
d. glacier.

The question asks for a specific time (the time of lower temperatures), so the answer can only refer to a time, not anything else, therefore we can exclude the answers "emigration" and "glacier" - the first is population movement and the second is a form of ice.

The correct answer is ice age-  a time of lower temperatures