Final answer:
Sauropods are the correct answer for large, four-legged plant eaters, as they include the largest herbivorous dinosaurs that ever lived, characterized by long necks and massive bodies.
Step-by-step explanation:
Sauropods are large, four-legged plant eaters. The dinosaurs in this classification were some of the largest land animals to have ever lived. Unlike therapsids, which were more closely related to mammals and showed a variety of body sizes and diets, sauropods were part of the Saurischia clade, which included the largest herbivorous dinosaurs known, such as Argentinosaurus. These majestic creatures had long necks, small heads, and massive bodies, which could reach up to 40 meters in length and weigh at least 80,000 kg. They thrived in the Mesozoic era, particularly during the Jurassic and Cretaceous periods.
Answering the question, '____ are large, four-legged plant eaters.':
a. Sauropods are the correct choice. Brachiopods are a group of marine invertebrates, theropods were mostly bipedal carnivores and included the ancestors of birds, and therapsids evolved into mammals.