Final answer:
The clade most closely related to birds within the coelurosaur tree is the group of theropod dinosaurs known as maniraptorans.
Step-by-step explanation:
Within the coelurosaur tree, the clade that is most closely related to birds is the theropod dinosaurs, specifically the maniraptoran theropods. The fossil record, including the well-known Archaeopteryx, provides evidence that birds evolved from these bipedal predators. Significant characteristics linking maniraptoran theropods and birds include the structure of the hip and wrist bones, the presence of the wishbone, and, of course, feathers.
The clade Neornithes, which encompasses the modern avian crown group, is considered the descendant of the most recent common ancestor shared with these maniraptoran theropods. Birds have undergone significant radiation and diversification since the extinction of their non-avian dinosaur relatives, resulting in the three major clades we see today: Paleognathae, Galloanserae, and Neoaves.