Final answer:
Sauropods are known for their immense size, long necks, specific limb and foot posture, and the structural design of their feet. These features collectively supported their lifestyle as some of the largest land animals that ever existed.
Step-by-step explanation:
The key aspects of Sauropods include all of the above options: 1) their massive size and the development of long necks as a preexisting feature, 2) their distinctive limb and foot posture, and 3) the unique structure of sauropod feet.
Sauropods were part of the dinosaur clade Saurischia, characterized by a simple, three-pronged pelvis, and they grew to be among the largest land animals to have lived. Their long necks likely evolved to help them reach high vegetation without having to move their massive bodies, while their limb and foot posture supported their enormous weight. The specific structure of their feet, with a broad, columnar shape, helped distribute their weight and prevent sinking into soft ground.