GMAT Sentence Correction (SC) | Re: Despite the fact that it has a duck-shaped bill and lays eggs, the pla

both idioms are correct
not x but y
not x but rather y

A) bird; rather - we need a contrast here and complete the idiom "not x but rather y". so "but" is required.
B) bird, but rather - correct.

the platypus is not a bird; but rather the mammal -- now we don't need "that of" here. C,D &
...

---