Final answer:
The evolutionary novelties of tyrannosaurids include very small forelimbs, distinguishing them from other theropod dinosaurs. Thus, the option A is the correct answer.
Step-by-step explanation:
The evolutionary novelties of tyrannosaurids include very small forelimbs. Tyrannosaurids, a family of theropod dinosaurs, are known for several distinct features. One of their most notable characteristics is the size of their forelimbs, which were extremely small relative to their large bodies. This contrasts with their robust hind limbs, which supported their bipedal stance. The incongruity of such tiny arms is still a topic of scientific curiosity. Although they had large claws and serrated teeth as well, these were not unique to tyrannosaurids and thus are not considered their hallmark evolutionary innovations.