GMAT Sentence Correction (SC) | Re: Although some ornithologists contend that the precursors of birds are

In the original sentence, the precursors - are has a subject verb mismatch (since the precursors is plural)
The verb which follows must be were(plural verb for plural subject)

Also the creatures that glide is not right. it must be glided.
Option D corrects both these errors and must be the answer choice

---