Constructing a phylogenetic tree for the evolution of birds?

c, probably.

No modern birds have teeth and only the ostrich and similar flightless birds lack a keeled sternum. Both reptiles and birds have claws. Many dinosaurs are now known to have had feathers.

c

Starting with the fact that Archaeopteryx is clearly a bird due to its wings and feathers, one needs to find features in it that do not belong to birds but are seen in dinosaurs. It had a flat sternum, so the last two are out. The first one is out, most dinosaurs had no wings so they do not link archyx back to the dinos.

Therefore it is between b and c. In c the flat sternum is not a feature of modern birds, nor are teeth, these link it to dinosaurs. All the features of b are true of modern birds and it seems also of some dinosaurs but to imply a linkage the features must be unique to dinosaurs. For example, some dinosaurs were bipeds and so are we but our common ancestor was a quadruped and bipedalism just indicates convergent evolution. This means b is ruled out and the answer is c.