Hunter-gatherer societies demonstrate the strongest dependence on the environment of the various types of preindustrial societies. As the basic structure of human society until about 10,000–12,000 years ago, these groups were based around kinship or tribes. Hunter-gatherers relied on their surroundings for survival—they hunted wild animals and foraged for uncultivated plants for food. When resources became scarce, the group moved to a new area to find sustenance, meaning they were nomadic. These societies were common until several hundred years ago, but today only a few hundred remain in existence, such as indigenous Australian tribes sometimes referred to as “aborigines,” or the Bambuti, a group of pygmy hunter-gatherers residing in the Democratic Republic of Congo. Hunter-gatherer groups are quickly disappearing as the world’s population explodes.
Q. The phrase these groups refers to …
D is correct because option a is a singular subject that does not refer to multiple groups while option b, “years ago”, does not refer to multiple groups of something. Although option c could fit and is grammatically correct, it is not the main subject to which the pronoun referent refers. In order to confirm that this is indeed the correct answer, you could look to the next sentence, which uses the original subject, “hunter-gatherers”, again.