Final answer:
The primary erosion that shaped the Grand Canyon occurred during the a. Pleistocene epoch.
Step-by-step explanation:
Most of the river erosion that formed the Grand Canyon occurred during the period known as the a. Pleistocene. The Pleistocene is the geological epoch that lasted from about 2,588,000 to 11,700 years ago, spanning the world's most recent period of repeated glaciations.
The Colorado River established its course through the Grand Canyon about six million years ago, during the Pliocene, and it is during the Pleistocene that the canyon deepened and widened largely due to river erosion.