Final answer:
The correct answer is (d) Sauropod, which includes dinosaurs like Argentinosaurus, some of the largest land animals to have ever lived.
Step-by-step explanation:
The largest animals ever to walk the earth were the sauropods, which makes option (d) Sauropod the correct answer to the question. Sauropods were known for their enormous size, with some members of this group like Argentinosaurus reaching lengths of nearly 40 meters (130 feet) and weighing at least 80,000 kg (88 tons). These dinosaur species were part of a diverse group of reptiles that were not lizards but had distinctive upright postures different from any other reptilian forms. The sauropods' large dimensions even challenge the size of the great blue whale, although whale weights can surpass those of the sauropod dinosaurs.