In 1860 Bavarian quarry workers discovered the impression of a feather

In 1860 Bavarian quarry workers discovered the impression of a feather in a limestone slab dating to the Mesozoic era. It had previously been assumed that birds developed only after the close of the Mesozoic era and after the disappearance of pterosaurs, a species characteristic of that era. But there in limestone lay the imprint of a fully aerodynamic, three-inch-long feather. This, therefore, must have been the earliest bird—certainly the earliest found to the date.

The argument assumes which one of the following?

As per the passage Pterosaurs was very common in Mesozoic era and birds developed near the end of this era and after the disappearance of pterosaurs. However, a fully aerodynamic three-inch-long feather was discovered in Mesozoic era limestone slab. Hence author concludes that the feather must have been that of the earliest bird. Clearly, author assumes that Pterosaurs or any other bird like animal didn't had an such sort of feature - feather.

(A) The creature to which the feather belonged was a descendant of the pterosaurs. - WRONG. Whether a descendant or not it does not matter since that slab belonged to Mesozoic era by the end of which pterosaurs disappeared.

(B) Birds with such feathers were preceded by species of birds with less-developed feathers. - WRONG. Irrelevant. Not concerned whether feather developed fully or not.

(C) In the Mesozoic era, no creatures other than birds had such feathers. - CORRECT. Since the argument talks about the earliest bird it can be said that no other creature, which might have developed before mesozoic era, had such feathers. Also, on negating we get that 'other creature had such feathers' which breaks the argument's claim that it must have been the earliest bird.

(D) The feather belonged to a Mesozoic creature that was neither a pterosaur nor a bird, but intermediate between them. - WRONG. If the feather belonged to an intermediate creature

(E) The earliest bird flew in an awkward manner. - WRONG. Irrelevant. Does n't impact the argument.

IMO Answer C.