One primitive trait of Ardipithecus ramidus is its:

Analyze the options: Evaluate each trait listed in the problem to determine whether it is primitive or derived. For example, 'fully bipedal locomotion like modern humans' is a derived trait, as it is a hallmark of later hominins. Similarly, 'short canine teeth similar to modern humans' and 'large brain size comparable to Homo sapiens' are also derived traits.

Focus on the opposable big toe: The opposable big toe adapted for grasping is a primitive trait because it is a feature seen in earlier primates, allowing them to grasp branches and climb trees. This trait is not present in modern humans, who have a non-opposable big toe adapted for bipedal walking.