Sentence Correction

Thanks for typing out the question, parallel_chase!

Rock samples
------<taken from R of an A>
--------------------------<about twice the size of (another) A>
--------------------------------------------------------------<that eradicated D>
has (have!) been dated... etc.

Hopefully that formatting is clear - the stuff in brackets modifies the noun above where each <> starts. So, first, we have that subject-verb agreement issue. Eliminate A and B.

Next, we have "have been dated... (to be) (as being) or (at)"

The idiom is "dated at." No great explanation - there never is for an idiom. It just is what it is and we're supposed to memorize it.

It might help you remember to think of it this way:
Someone dated something AT a certain date - right now, they know (or claim) that it is this certain date.

If someone dated something TO BE a certain date, it could be interpreted as: what it will be in the future - it's going to be that date.

Or, you can just memorize it.

Eliminate C and D. That leaves E.

(And good catch to those who pointed out that D is missing the verb "are" - it's easy to miss little things like that. We can eliminate B for that same reason, and we can also eliminate A for a bad subj-verb match (is).