Final answer:
During glacial retreat, the c)melting and shrinking of the glacier's extent occur, as observed with the glaciers shrinking in Glacier National Park due to warmer temperatures and climate change.
Step-by-step explanation:
During glacial retreat, the process that occurs is melting and shrinking of the glacier's extent. This happens when the rate of ice melting exceeds the accumulation of snow. As seen in the case of Glacier National Park in Montana, where glaciers have been retreating due to climate warming, this process has led to a significant decrease in the number and size of glaciers over time. Although the formation of moraines can occur as glaciers deposit rocks and soil at their margins, this is not a process that happens during the retreat but a result of past glacier movement. Similarly, the accumulation of snow and ice at the glacier's terminus and advancement of the glacier over the landscape are processes related to glacier growth, not retreat.