Final answer:
The theropod group known to be herbivores is the Therizinosauria, characterized by their unique physical adaptations for a plant-based diet.
Step-by-step explanation:
The theropod group that is generally considered to consist entirely of herbivores is the Therizinosauria. These theropods, characterized by their long necks, large gut cavities, and leaf-shaped teeth, are notable for their plant-eating lifestyle. This is in contrast to the typical portrayal of theropods as fierce predators like Tyrannosaurus rex. Therizinosaurs were bipedal and possessed large, curved claws, which may have been used for grabbing branches and pulling down vegetation for feeding.