GMAT Sentence Correction (SC) | Re: Sixty-five million years ago, according to some scientists,

In D, "an event that....extinctions" is set off by commas from the rest of the sentence and provides extra information about the event (the asteroid slammed into NA). So you should be able to remove this portion of the sentence without changing the meaning of the sentence.

Let's readD:
[Modifiers to provide context] An asteroid slammed into NA,[extra information about this event] , and it marks the end...

Now ask what "it" refers to.

If you want "marks the end..." to refer to the event, then "that" must replace "it":
An
...

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