Which prehistoric animal is characterized as having long tusks and is considered a relative of modern elephants?

The woolly mammoth is the correct answer, as it is recognized for its long tusks and is a close relative of modern elephants. Woolly mammoths belonged to the family Elephantidae, which also includes today's elephants, and they thrived during the Pleistocene Epoch. They were well-adapted to cold environments, with thick fur and layers of fat for insulation, alongside their impressive curved tusks which they used for foraging under the snow and for combat during mating displays or territorial disputes.

On the other hand, mastodons, while similar in being large, tusked prehistoric mammals, belong to a different genus than woolly mammoths and have distinct features such as their molars, which were suited for a diet of leaves and branches rather than grasses. The giant beaver, although massive in size, is not related to elephants and did not exhibit tusks as a defining characteristic. Similarly, the saber-tooth cat is a completely different type of prehistoric predator, known for its elongated canine teeth, and does not share any direct relation to elephants or tusked animals. The woolly mammoth, with its defining features, stands out as the appropriate answer.