Final answer:
Equestrian foraging societies existed in the Great Plains of North America and the grasslands of Southern Argentina due to the reintroduction and adoption of horses, which transformed the lifestyle of peoples like the Sioux and Comanche into nomadic hunter/gatherers.
Step-by-step explanation:
Equestrian foraging societies existed in several regions around the world. The culture of many Plains peoples, such as the Sioux, Crow, Cheyenne, Pawnee, and Comanche, transformed with the reintroduction of the horse to North America at European contact. They quickly adopted the use of horses in following and hunting the great bison herds and changed their way of life from farmers to nomadic hunter/gatherers, becoming one of the most important groups in the northern Plains region. Similarly, the grasslands of Southern Argentina also supported nomadic foraging societies. Therefore, the correct answer to the question about where equestrian foraging societies existed is 'd) b and c' which refers to the Great Plains of North America and the grasslands of Southern Argentina.