The geologists declared that the fossil was authentic.

Answer: Option B

Authentic : of undisputed origin and not a copy; genuine.
Old : having lived for a long time; no longer young.
Fake : not genuine; imitation or counterfeit.
New : produced, introduced, or discovered recently or now for the first time; not existing before.
Beautiful : pleasing the senses or mind aesthetically.

Antonym of Authentic is Fake