Archeologists in the Sudan have unearthed a bone of a

Yet another OG counterfeit.

OA is A

OE:

Look for pronouns in the underlined part of the sentence. They indicates that pronouns are likely being tested.

Rule and Fix:
A Pronoun must clearly and correctly refer to only one thing and must agree in gender and number with that thing. They clearly refers to and agrees with Archeologists.

POE:
All choices use they so nothing can be eliminated.

Chunk and Compare:
Compare A and B use assume to be the other choices use assume is. This is an idiom hopefully you have memorized this idiom. The correct idiom is assume to be.

Eliminate B, C, and D.

Compare A to E.
E uses the pronoun it but A does not. Since it should refer to bones it should be plural. Reread your choice:
Choose A.

I agree with A.

Since the modifier 'one' is already present, we do not need another modifer 'that'. So D and E are out

A is idiomatic with 'assume to be'