Sixty-five million years ago, according to some scientists, an asteroid bigger than Mount Everest slammed into North America, which, causing plant and animal extinctions, marks the end of the geologic era known as the Cretaceous Period.
A. which, causing plant and animal extinctions, marks
B. which caused the plant and animal extinctions marking
C. and causing plant and animal extinctions that mark
D. an event that caused plant and animal extinctions, and it marks
E. an event that caused the plant and animal extinctions that mark

OA is E. Can you please explain why D is wrong.

Seems that you have narrowed it down to D & E.

D is wrong for two reasons

(1) it lacks clear antecedent.

(2) Its not the event (assuming it refers to event) that mark the end of CP, but the extinction

If you use option D, this is what you've got, stripped down:

...an asteroid (description of asteroid) slammed (info about where it slammed), description of a consequence of the event just mentioned, and it marks...

more simply (stripping out all of the modifiying stuff):
an asteroid slammed, and it marks...

So, the asteroid marks the end of...?

Not the asteroid itself. The fact that the asteroid slammed into the Earth and caused extinctions. But the sentence just says "the asteroid marks..."

Further, because the word "and" separates these two parts of the sentence, the part before and the part after are completely separate. I went to the grocery store and exercised today. Did I exercise in the store? No. These are two separate actions that I took today.

So do you want the info connected - are the events before the "and" connected to the events that happened after the "and"? Or not? Yes, they're connected: the asteroid --> extinctions --> end of an era. So we don't want to separate the two halves with the word "and" because we then lose that connection. So D is no good.

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Stacey Koprince
Instructor
Director of Online Community
ManhattanGMAT