Options A and B are out because "which" refers to North America incorrectly.

Option C is out because "and causing" isn't parallel to the past tense "slammed" that appears earlier.

That leaves D and E, both which begin with "an event." Modifiers that begin with a subject like this are tricky. They are not limited to modifying the immediately preceding word. They can describe any noun that came before it or even the entire preceding clause. Here it describes the entire clause "an asteroid slammed...".

Between the two, E is the right answer because D uses the singular pronoun "it" to refer to extinctions.