Which of the following is a synapomorphy linking dinosaurs and birds?
a. loss of teeth
b. furcula
c. small size
d. endothermy

b. Furcula

Explanation:

A synapomorphy is a new characteristic (a novelty in terms of biological evolution) that appears in a linage and is inherited in its descendants.

Under the hypothesis that states that birds are the surviving descendants of dinosaur's linage, evolutionary biologists and paleontologists try to provide evidence to support the linking between both groups. If the relation between these group is an ancestor (dinosaur)-descendant (birds) relationship, one would expect to find at least one shared biological feature that supports it.      

It is documented in the fossil record that dinosaurs have a well developed set of specialized teeth (whether they were carnivorous or herbivorous).  

Birds do not have any teeth, while having a corneous beak .  

Also, although there were small-sized dinosaurs, we have great evidence of huge living forms  (think in T-Rex, Brachiosaurus or Carnotaurus), that have nothing to do with birds's sizes.    

As far as endothermy, the ability of maintaining a constant temperature (independently of the environment), this feature is present in birds but not clearly demonstrated for dinosaurs.

On the other hand, both dinosaurs (at least the group of non-avian theropods) and birds do share the characteristic of having a furcula, a clavicle-like forked bone that is of great importance to facilitate extant birds's flight.  Fossil record has provided key evidence of the presence of furcula in dinosaurs lineages and birds ancestors (like Archaeopteryx).